Serverpod uses FlutterFire UI to handle authentication through Firebase. It allows you to add social sign-in types that Serverpod doesn't directly support.
To add authentication with Firebase, you must first install and initialize the Firebase CLI tools and Flutter fire. Follow the instructions here for your Flutter project. In the Firebase console, configure the different social sign-ins you plan to use. Then pass your provider configurations to either the signInWithFirebase method or the SignInWithFirebaseButton of the serverpod_auth_firebase_flutter package.
Server-side, you need to create a set of authentication keys in the Firebase console, download the JSON file, rename it to firebase_service_account_key.json and place it in the config folder.
Serverpod automatically merges accounts that are using the same email addresses, so make sure only to allow sign-ins where the email has been verified.