react-native-firebase/docs/core/app.md

App - firebase.app(): FirebaseApp

RNFirebase supports both initializing apps natively and also via js code over the RN bridge.

Apps initialized natively are available immediately at app runtime, there is no need to call initializeApp for them.

For example, to access the default app initialized via the Google-Services plist or json file:

import firebase from 'react-native-firebase';

const defaultApp = firebase.app();

defaultApp.database().ref('foobar').once('value', (snapshot) => {
  // snapshot from default app
});

// get the default app name/options that were initialized natively
console.log("name", defaultApp.name);
console.log("apiKey", defaultApp.options.apiKey);
console.log("applicationId", defaultApp.options.applicationId);
console.log("databaseUrl", defaultApp.options.databaseUrl);
console.log("messagingSenderId", defaultApp.options.messagingSenderId);
console.log("projectId", defaultApp.options.projectId);
console.log("storageBucket", defaultApp.options.projectId);