architecture-samples VS reactive-mvvm-android

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

I would definitely avoid putting everything in the Activity. Usually you want to separate your code between multiple layers like data, domain and view. Then you can choose a pattern like MVVM, MVI or MVP (MVP is not used that much anymore). Here you have a repo I made with a reactive MVVM example: Github Reactive MVVM with Flows