How do I add more fields in Firebase authentication?

If you want to add additional data, you need to create a model class for your user and then store it in your Firebase database . In general, after sign up, you can create a node tree for user like picture. Then, you can get the info with user key that can get by calling FirebaseUser.

Get Started with Firebase Authentication on Websites

You can integrate Firebase Authentication with a custom authentication system by modifying your authentication server to produce custom signed tokens when a user successfully signs in . Your app receives this token and uses it to authenticate with Firebase.