Is it possible to generate a push notification (if the app is not in the foreground) on firebase db changes? I started coding an android app.
There certainly is. Just have your Android app write the notification request into the database.
Sending device-to-device notifications is not a supported scenario in Firebase Cloud Messaging (see this answer), so you'll need a server component for that. For an example of how to send a push notification in nodejs, see this answer.
Firebase provides a tool called Cloud Functions.
With this, you can listen to your Firebase Database and send Notifications without the need for an external server :)
See : https://firebase.google.com/docs/functions/use-cases
To do it in a simple way, you can use a Service, in this Service you can set a Listener on a ref in your firebase DB when the event is triggered you can create a notify and show it to the user.
This is a very simple tutorial: https://www.simplifiedcoding.net/android-push-notification-tutorial-using-firebase/