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