To create the main APKThe Android Package format output by Unity. An APK is automatically deployed to your device when you select File > Build & Run. More info
See in Glossary 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 SceneA Scene contains the environments and menus of your game. Think of each unique Scene file as a unique level. In each Scene, you place your environments, obstacles, and decorations, essentially designing and building your game in pieces. More info
See in Glossary is available. In this case, you should attempt to download the APK expansion file. For more information, see Download process.