how to stop apps running in the background

You can stop background apps by restricting their battery usage in your device settings or by using built-in power-saving modes.

Restrict Individual Apps. Go to Settings > Apps, select the specific app, and navigate to Battery or App battery usage. Set the app to Restricted or toggle off Allow background usage to prevent it from running processes when not in use.
Use System-Wide Battery Settings. Enable Battery Saver mode in Settings > Battery to limit background activity across the entire device. Some manufacturers, like Samsung, offer additional tools like Deep Sleeping apps to prevent specific apps from running in the background.
Uninstall or Disable Unused Apps. If you no longer need an app, uninstall it. For pre-installed system apps that cannot be uninstalled, you can select Disable in the app's settings menu to stop it from running entirely.
Manage Background Refresh on iPhone. On iPhone, navigate to Settings > General > Background App Refresh. You can turn this feature off completely or toggle it individually for specific apps to prevent them from refreshing content in the background.