Android 'Add-on' for app
Without having looked more closely at your example, the "add-on" is likely just an app that doesn't have any activities declared as launcher activity. It doesn't show up on your home screen, because Android doesn't know what it would have to show you if you tried to open it.

The main app can then check if you have the add-on app installed, and communicate however it needs to with it.

There's no additional black magic here, just a regular app that isn't meant to be opened by the user directly.