When a user signs in, your React app requests a JWT from the authentication server. The authentication server uses a private key to construct a JWT, which it then delivers back to your React app. When your user wants to make a request, your React app saves this JWT and delivers it to your backend server.
All users can be saved, and you can log in using Google, Facebook, or email. It's similar to an online database, and it's simple to connect to it from Android Studio, where you can then login with the users.
Check to see if you're logged into your Google account. Give your project a name. I'll call it firebase-auth-article in my case. After you've given it a cute name, click Continue, and you should be given the choice to turn on Google Analytics.