kotlin-android-tutorial | Android Application which can serve as a tutorial | Android library

I have to be honest I don't understand at all how to specify which database I'm talking to. Is FireStore to be considered one ginourmous database, with the documents and collections of all my applications living side by side?

Do I have to give app specific names for the collections... say AwesomeAppUsers, and ThatOtherAppUsers, and AnotherAppUsers? Or what? Just mutually mash together users under "/users"?

I'm reading through the Firebase documentation, and all I'm seeing are lines of code referring to a particular collection, or document, but none that refers to any namespaces, or databases.

There's no setup, no config for where it should be pointed, you just sign in with firebase, and then you run

firebase deploy --only functions

And then magic.

Can someone please demagicify this, and explain me how an app can communicate with specific functions, when there's no setup?

The worst is when I see this in app writing guides. Like here http://resocoder.com/2018/05/25/firebase-firestore-chat-app-cloud-functions-fcm-ep-8-kotlin-android-tutorial/, two different repositories, a backend and an app... how does the app know which of the quintillion backend functions you'll be writing to subscribe to?