Chat app on Android

I would like to create a chat app on Android.
However, I do not know the basic configuration.
I think I should use Firebase.

realtime database and
Firebase Cloud Messaging Android Are there two?

If you use the realtime database, you will see the same chat screen between the two Android devices.However, if someone using one device closes activity, the other person doesn't know if activity has been updated, so I think they need to notify them via Firebase Cloud Messaging Android that the chat is on.

I would like to know how to make a simple message and notify the other person when they are chatting and one person closes the screen.Of course, if you don't close the screen and see it properly, you won't receive a simple message.

realtime database
Firebase Cloud Messaging Android

I think both of them are a little different.