Push notifications

The app asks the user for permission, and the operating system gives it a device token, an address for that app on that device. The app sends the token to your server. To notify the user, the server sends a message with the token to the platform's push service (APNs for Apple devices, FCM for Android, the browser's push service on the web), which delivers it and wakes the app if needed.

Permission is the gate: iOS has always asked, Android asks from version 13, and people who get too many alerts switch them off or uninstall. Messages can be visible notifications or silent data messages that update the app in the background, and delivery is not guaranteed. Services such as OneSignal and Expo's push service wrap the platforms behind one API.