This page describes how APK expansion files work in the context of a Unity Android application.
Unity can automatically split an application into a main APK and a single APK expansion file that uses the .obb file extension. In Unity, this process is called splitting the application binary. The APK expansion file this process generates is the main APK expansion file and if you want to create the patch APK expansion, you must do that manually.
To create the main APK expansion file for your application, indicate to Unity to split the application. For information on how to do this, refer to 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, try to download the APK expansion file. For more information, refer to Download process.
Unity only automatically creates the main APK expansion file, but you can manually create the patch APK expansion file if the application requires more storage space. For information on how to create the patch APK expansion file, refer to Android’s Development checklist documentation.
When you create the patch expansion file, you must include a unity_obb_guid file within it so Android recognizes the APK expansion as being compatible with the APK. To do this:
unity_obb_guid file.