Understanding client-side flow of Firebase messaging

I'm trying to dive deep into the client side flow of a FCM message (once the message is received on a Android device).

I understand that Android has some native Firebase system service that initially receives the Firebase message (if device not offline). How is this message delivered to the intended application (using broadcasts, intent?)? How does this native Firebase service know which application to send the message to?

It is mentioned in the docs that the FirebaseMessagingService on the device can receive the Firebase message even when stopped. How does it work in this case?

In an application, one can have a single FirebaseMessagingService but multiple FirebaseApp instances. How do these different FirebaseApp instances for an app factor into message delivery?

I'd appreciate a detailed understanding of the flow, I haven't been able to find any official docs/blogs on this.

from Understanding client-side flow of Firebase messaging