Building Android apps at the intermediate level?

Question

Android

Hello! I want to learn how to write applications on Android. Let me emphasize right away that I want to learn for MYSELF . I am not pursuing the moronic idea of ​​writing a super mega hit app for earning millions of oil on Google Play alone.
I have good knowledge of Java (web design). Installed Android Studio. Wrote my first "Hello World"-type application. And on this everything stalled. I couldn't find good tutorials. There are old tutorials dating back to 2010, a la " let's write a ninja game ". And the new tutorials are just a repetition of Java and "Hello World" again.
I have a great desire to write a good application (no matter how much time it takes, a month or a quarter). I can draw well. I know Java. But I can't find good tutorials or books . I have a lot of time. And I know my abilities.
The question itself : how to write a good application tied to my store (purchases, notifications, etc.) and how to write a game in 3D (although the second question is not in a hurry, it's just for fun, but probably it should be addressed to Unity).
Thanks in advance for your replies friends!

Answer 1

I can recommend Devcolibri. There is now starting a course on creating a working application with web services from A to Z. https://www.youtube.com/user/devcolibri/playlists

Answer 2

Couldn't you get straight to the point? The rest of the local inhabitants are hardly interested.
1. The question is too general, write a small (Rest) API in your store to interact with the application and then follow the client-server principle.
2. Unity3d is the first thing that came to mind, and the rest of the information (DOFIGA) is in the search