What to test in an android application?

Question

Android

Hello!
I study android development in courses with udemy. As it turned out, those courses are not enough. The question arose about testing written applications, but I have no idea how to do this. In the Russian-speaking segment of the Internet, information is scarce. There is more information in the English-speaking segment, but, for some reason, I can’t attach it to my project. There is such a simple application https://github.com/cjybxrf/ImagesApp
Tell me what to test in it. Maybe it didn’t work out to comply with the MVVM architecture and the tests are basically impossible? I read that they test viewModel and model, but what to test there and how I don’t understand. I would like, if you don't mind, of course, an example code of 2 tests. So that I understand with an example what can be done with this code.

Answer 1

Wishing to help me and a dozen is not recruited. I would even say couples.