Updated tutorial: https://www.b4x.com/android/forum/threads/b4x-firebase-push-notifications-2023.148715/ Clarification: The nice thing about FCM is that your app doesn't need to run in order to receive messages. The FirebaseMessaging receiver will be started by the OS when a new message...
You can use notifications through firebase.
When the user logs in to your app, you capture the user's unique firebase token, and send it to your server to save the login data with the token. When you want to send a message to the user, you search your database for that user's token and put a filter on the firebase message, so that it is sent only to that user.