architecture-samples VS Mona-Android-App

Here's an example project on GitHub that demonstrates how to use a ViewModel in practice: https://github.com/android/architecture-samples/tree/master/BasicSample

For me this one is the best: Architecture samples it's easy to understand and covers lots of concepts.

Since I never had a mentor or any courses I decided to follow anyone in their development approaches and my choice was google team. There are repo which is supported and update for years, it changes and might slightly differ in branches. But it could answer your questions.

For merging multiple data-sources, like network and database, I can use Repository backed with NetworkDataSources and DbDataSources. For data manipulation, Kotlin's collection has many useful functions (map, reduce, groupBy, count, etc.). If I want to ensure a data can only be emitted once, a simple modification to LiveData is enough (SingleLiveData

Take a look on Android blueprints https://github.com/android/architecture-samples

The stable version of Jetpack Compose is just released last year. While I do like writing UI using Compose, I think most companies still need more time to adopt Compose.