Before you can add Firebase to your Flutter app, you need to create a Firebase project to connect to your app. Visit the link below to learn more about Firebase projects.
If you're releasing your Flutter app on both iOS and Android, register both the iOS and Android versions of your app with the same Firebase project.
Visit for more information on how to setup for Android
Visit for more information on how to setup for iOS
Add Firebase to your Flutter app  | Firebase Documentation
2. Enter your app's package name in the Android package name field.
Make sure that you enter the ID that your app is actually using. You cannot add or modify this value after you register your app with your Firebase project. Both applicationId should be matched.
3. Click Register app.
Add a Firebase configuration file
Click Download google-services.json to obtain your Firebase Android config file (google-services.json).
Move your config file into the android/app directory of your Flutter app.
Follow the instruction for enabling Firebase services in your Android app