By default, every application runs in its own Linux process. Android starts the process when any of the application's components need to be executed, then shuts down the process when it's no longer needed or when the system must recover memory for other applications.

protected void onListItemClick(ListView l, View v, int position, long id) {
        Intent intent = intentForPosition(position);
        startActivity(intent);
    }

@Override
    public void startActivity(Intent intent) {
        this.startActivity(intent, null);
    }

// We now tell the activity manager it is okay to run third party
        // code.  It will call back into us once it has gotten to the state
        // where third party code can really run (but before it has actually
        // started launching the initial applications), for us to complete our
        // initialization.
        mActivityManagerService.systemReady(new Runnable() {
            @Override
            public void run() {
                Slog.i(TAG, "Making services ready");
                mSystemServiceManager.startBootPhase(
                        SystemService.PHASE_ACTIVITY_MANAGER_READY);

                try {
                    mActivityManagerService.startObservingNativeCrashes();
                } catch (Throwable e) {
                    reportWtf("observing native crashes", e);
                }

                Slog.i(TAG, "WebViewFactory preparation");
                WebViewFactory.prepareWebViewInSystemServer();

                try {
                    startSystemUi(context);
                } catch (Throwable e) {
                    reportWtf("starting System UI", e);
                }
                try {
                    if (networkScoreF != null) networkScoreF.systemReady();
                } catch (Throwable e) {
                    reportWtf("making Network Score Service ready", e);
                }
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                try {
                    if (networkManagementF != null) networkManagementF.systemReady();
                } catch (Throwable e) {
                    reportWtf("making Network Managment Service ready", e);
                }
                try {
                    if (networkStatsF != null) networkStatsF.systemReady();
                } catch (Throwable e) {
                    reportWtf("making Network Stats Service ready", e);
                }
                try {
                    if (networkPolicyF != null) networkPolicyF.systemReady();
                } catch (Throwable e) {
                    reportWtf("making Network Policy Service ready", e);
                }
                try {
                    if (connectivityF != null) connectivityF.systemReady();
                } catch (Throwable e) {
                    reportWtf("making Connectivity Service ready", e);
                }
                try {
                    if (audioServiceF != null) audioServiceF.systemReady();
                } catch (Throwable e) {
                    reportWtf("Notifying AudioService running", e);
                }
                Watchdog.getInstance().start();

                // It is now okay to let the various system services start their
                // third party code...
                mSystemServiceManager.startBootPhase(
                        SystemService.PHASE_THIRD_PARTY_APPS_CAN_START);

                try {
                    if (wallpaperF != null) wallpaperF.systemRunning();
                } catch (Throwable e) {
                    reportWtf("Notifying WallpaperService running", e);
                }
                try {
                    if (immF != null) immF.systemRunning(statusBarF);
                } catch (Throwable e) {
                    reportWtf("Notifying InputMethodService running", e);
                }
                try {
                    if (locationF != null) locationF.systemRunning();
                } catch (Throwable e) {
                    reportWtf("Notifying Location Service running", e);
                }
                try {
                    if (countryDetectorF != null) countryDetectorF.systemRunning();
                } catch (Throwable e) {
                    reportWtf("Notifying CountryDetectorService running", e);
                }
                try {
                    if (networkTimeUpdaterF != null) networkTimeUpdaterF.systemRunning();
                } catch (Throwable e) {
                    reportWtf("Notifying NetworkTimeService running", e);
                }
                try {
                    if (commonTimeMgmtServiceF != null) {
                        commonTimeMgmtServiceF.systemRunning();
                    }
                } catch (Throwable e) {
                    reportWtf("Notifying CommonTimeManagementService running", e);
                }
                try {
                    if (textServiceManagerServiceF != null)
                        textServiceManagerServiceF.systemRunning();
                } catch (Throwable e) {
                    reportWtf("Notifying TextServicesManagerService running", e);
                }
                try {
                    if (atlasF != null) atlasF.systemRunning();
                } catch (Throwable e) {
                    reportWtf("Notifying AssetAtlasService running", e);
                }
                try {
                    // TODO(BT) Pass parameter to input manager
                    if (inputManagerF != null) inputManagerF.systemRunning();
                } catch (Throwable e) {
                    reportWtf("Notifying InputManagerService running", e);
                }
                try {
                    if (telephonyRegistryF != null) telephonyRegistryF.systemRunning();
                } catch (Throwable e) {
                    reportWtf("Notifying TelephonyRegistry running", e);
                }
                try {
                    if (mediaRouterF != null) mediaRouterF.systemRunning();
                } catch (Throwable e) {
                    reportWtf("Notifying MediaRouterService running", e);
                }

                try {
                    if (mmsServiceF != null) mmsServiceF.systemRunning();
                } catch (Throwable e) {
                    reportWtf("Notifying MmsService running", e);
                }
            }
        });

public void systemReady(final Runnable goingCallback) {
        ...
        // Start up initial activity.
        mBooting = true;
        startHomeActivityLocked(mCurrentUserId, "systemReady");
		...
    }

boolean startHomeActivityLocked(int userId, String reason) {
        if (mFactoryTest == FactoryTest.FACTORY_TEST_LOW_LEVEL
                && mTopAction == null) {
            // We are running in factory test mode, but unable to find
            // the factory test app, so just sit around displaying the
            // error message and don't try to start anything.
            return false;
        }
        Intent intent = getHomeIntent();
        ActivityInfo aInfo =
            resolveActivityInfo(intent, STOCK_PM_FLAGS, userId);
        if (aInfo != null) {
            intent.setComponent(new ComponentName(
                    aInfo.applicationInfo.packageName, aInfo.name));
            // Don't do this if the home app is currently being
            // instrumented.
            aInfo = new ActivityInfo(aInfo);
            aInfo.applicationInfo = getAppInfoForUser(aInfo.applicationInfo, userId);
            ProcessRecord app = getProcessRecordLocked(aInfo.processName,
                    aInfo.applicationInfo.uid, true);
            if (app == null || app.instrumentationClass == null) {
                intent.setFlags(intent.getFlags() | Intent.FLAG_ACTIVITY_NEW_TASK);
                mStackSupervisor.startHomeActivity(intent, aInfo, reason);
            }
        }

        return true;
    }

Intent getHomeIntent() {
        Intent intent = new Intent(mTopAction, mTopData != null ? Uri.parse(mTopData) : null);
        intent.setComponent(mTopComponent);
        if (mFactoryTest != FactoryTest.FACTORY_TEST_LOW_LEVEL) {
            intent.addCategory(Intent.CATEGORY_HOME);
        }
        return intent;
    }

void startHomeActivity(Intent intent, ActivityInfo aInfo, String reason) {
        moveHomeStackTaskToTop(HOME_ACTIVITY_TYPE, reason);
        startActivityLocked(null /* caller */, intent, null /* resolvedType */, aInfo,
                null /* voiceSession */, null /* voiceInteractor */, null /* resultTo */,
                null /* resultWho */, 0 /* requestCode */, 0 /* callingPid */, 0 /* callingUid */,
                null /* callingPackage */, 0 /* realCallingPid */, 0 /* realCallingUid */,
                0 /* startFlags */, null /* options */, false /* ignoreTargetSecurity */,
                false /* componentSpecified */,
                null /* outActivity */, null /* container */,  null /* inTask */);
        if (inResumeTopActivity) {
            // If we are in resume section already, home activity will be initialized, but not
            // resumed (to avoid recursive resume) and will stay that way until something pokes it
            // again. We need to schedule another resume.
            scheduleResumeTopActivities();
        }
    }

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    >

    <ListView
        android:id="@android:id/list"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        />

    <TextView
        android:id="@android:id/empty"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:gravity="center"
        android:text="@string/activity_list_empty"
        android:visibility="gone"
        android:textAppearance="?android:attr/textAppearanceMedium"
        />

</FrameLayout>

@Override
    protected void onCreate(Bundle icicle) {
        super.onCreate(icicle);
        
        mPackageManager = getPackageManager();

        if (!mPackageManager.hasSystemFeature(PackageManager.FEATURE_WATCH)) {
            requestWindowFeature(Window.FEATURE_INDETERMINATE_PROGRESS);
            setProgressBarIndeterminateVisibility(true);
        }
        onSetContentView();

        mIconResizer = new IconResizer();
        
        mIntent = new Intent(getTargetIntent());
        mIntent.setComponent(null);
        mAdapter = new ActivityAdapter(mIconResizer);

        setListAdapter(mAdapter);
        getListView().setTextFilterEnabled(true);

        updateAlertTitle();
        updateButtonText();

        if (!mPackageManager.hasSystemFeature(PackageManager.FEATURE_WATCH)) {
            setProgressBarIndeterminateVisibility(false);
        }
    }

@Override
    protected void onListItemClick(ListView l, View v, int position, long id) {
        Intent intent = intentForPosition(position);
        startActivity(intent);
    }

protected Intent intentForPosition(int position) {
        ActivityAdapter adapter = (ActivityAdapter) mAdapter;
        return adapter.intentForPosition(position);
    }

public Intent intentForPosition(int position) {
            if (mActivitiesList == null) {
                return null;
            }

            Intent intent = new Intent(mIntent);
            ListItem item = mActivitiesList.get(position);
            intent.setClassName(item.packageName, item.className);
            if (item.extras != null) {
                intent.putExtras(item.extras);
            }
            return intent;
        }

public static final String CATEGORY_HOME = "android.intent.category.HOME";

/**
     * The main entry point from zygote.
     */
    public static void main(String[] args) {
        new SystemServer().run();
    }

public static void main(String argv[]) {
        try {
            RuntimeInit.enableDdms();
            // Start profiling the zygote initialization.
            SamplingProfilerIntegration.start();

            boolean startSystemServer = false;
            String socketName = "zygote";
            String abiList = null;
            for (int i = 1; i < argv.length; i++) {
                if ("start-system-server".equals(argv[i])) {
                    startSystemServer = true;
                } else if (argv[i].startsWith(ABI_LIST_ARG)) {
                    abiList = argv[i].substring(ABI_LIST_ARG.length());
                } else if (argv[i].startsWith(SOCKET_NAME_ARG)) {
                    socketName = argv[i].substring(SOCKET_NAME_ARG.length());
                } else {
                    throw new RuntimeException("Unknown command line argument: " + argv[i]);
                }
            }

            if (abiList == null) {
                throw new RuntimeException("No ABI list supplied.");
            }

            registerZygoteSocket(socketName);
            EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_START,
                SystemClock.uptimeMillis());
            preload();
            EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_END,
                SystemClock.uptimeMillis());

            // Finish profiling the zygote initialization.
            SamplingProfilerIntegration.writeZygoteSnapshot();

            // Do an initial gc to clean up after startup
            gcAndFinalize();

            // Disable tracing so that forked processes do not inherit stale tracing tags from
            // Zygote.
            Trace.setTracingEnabled(false);

            if (startSystemServer) {
                startSystemServer(abiList, socketName);
            }

            Log.i(TAG, "Accepting command socket connections");
            runSelectLoop(abiList);

            closeServerSocket();
        } catch (MethodAndArgsCaller caller) {
            caller.run();
        } catch (RuntimeException ex) {
            Log.e(TAG, "Zygote died with exception", ex);
            closeServerSocket();
            throw ex;
        }
    }