Pokemon on Android explain to me like I am 5.

How to play Pokemon on my Android. Explain to me like I am 5 pls.

I saw a lot of people playing Pokemon on their devices but I don't get it.

I can't download it on the Play store. So how do I do it?

I don't wanna get any malware or something.

Do I need to have the game physically or just download it? I would buy it, if it works.

Sorry if my question is dumb.