Where is learning Android programming?

I am a coder, I have a website about Apk https://allapk.co. Now I want to create an android application like google play store but especially my store has apps that are not available on Google Play (eg, restricted country, removed from google play). I have tried to read through the knowledge related to android programming on the internet, but maybe that is not enough, I would like to take a course. Please let me know where I can learn android programming.