Android Setup
To allow the Android app to securely connect to your Firebase project, a configuration file must be downloaded and added to your project.
Generating Android credentials
On the Firebase console, add a new Android application and enter your projects details. The "Android package name" must match your local projects package name which can be found inside of the manifest tag within the /android/app/src/main/AndroidManifest.xml file within your project.
Download the google-services.json file and place it inside of your project at the following location: /android/app/google-services.json.
iOS Setup
To allow the iOS app to securely connect to your Firebase project, a configuration file must be downloaded and added to your project.
Generating iOS credentials
On the Firebase console, add a new iOS application and enter your projects details. The "iOS bundle ID" must match your local project bundle ID. The bundle ID can be found within the "General" tab when opening the project with Xcode.
Download the GoogleService-Info.plist file and place it inside of your project at the following location: /ios/GoogleService-Info.plist
For further details please refer to this amazing article on React Native Firebase.