To create the main APK expansion file for your application, indicate to Unity to split the application. For information on how to do this, see Splitting the application binary.
Now when you Build the application, Unity generates the APK and the main APK expansion file then copies them both to the output directory. Unity uses the name of the application followed by .main for the name of the APK expansion file. For example, if the application is called my-app, the APK will be my-app.apk and the APK expansion file will be my-app.main.obb.
If the application starts and it can’t find and load the main APK expansion file, only the first Scene is available. In this case, you should attempt to download the APK expansion file. For more information, see Download process.