For online applications, a database is needed to connect multiple devices. So we use Firebase database for that. You need to download the google service for every type of application. For the android application, download the google-services.json for android in Firebase database. After that, you have to copy the json file into android/app directory and replace the existing google-services.json. For the iOS application, download the GoogleService-Info.plist for iOS in Firebase database. After that, you have to copy the json file into the ios folder, and replace the old one.