You can easily sideload an app, sure. But recent version of Android restricted background running of the apps, making developers dependent on push notifications, and FCM notifications won't work without Google services.
And then maybe "what app a user selects", if the user manages to follow the procedure.
The only way to run an app non-stop is to have a persistent notification. And even then, apps are killed off left and right by the likes of Samsung. See https://dontkillmyapp.com for more details