In which thread does the standard service in Android run?

Main Thread.

The standard service runs in the same process as the application and, by default, performs its tasks on the main thread, risking blocking the UI. To perform long-running operations, the service should create and manage separate threads independently.

Example of creating and using a separate thread in a service:

public class MyService extends Service {

    private ExecutorService executorService;

    @Override
    public void onCreate() {
        super.onCreate();
        // Create a thread pool for executing tasks
        executorService = Executors.newSingleThreadExecutor();
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        // Perform a long operation in a separate thread
        executorService.submit(() -> {
            // Long operation, e.g., data loading
            Log.d("MyService", "Performing background task");
            try {
                Thread.sleep(5000); // Simulate work
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            Log.d("MyService", "Background task finished");
            // Optionally notify the UI about task completion
        });
        return START_STICKY; // Or another appropriate flag
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        // Shutdown the thread pool when the service is destroyed
        executorService.shutdown();
        Log.d("MyService", "Service destroyed");
    }

    @Override
    public IBinder onBind(Intent intent) {
        return null;
    }
}

An alternative solution for background tasks is to use IntentService (deprecated) or modern solutions like WorkManager or Coroutines, which manage threads automatically.