Newbie with a looot of doubts

That’s the best way to learn. Welcome.

Yes.

No. But you might want to once you discover the possibilities.

I don’t use smarthings so someone else might comment on this. But I believe the post below answers your question.

You can indeed control devices through your xiaomi gateway as well.

Think of Home Assistant as a hub for hubs. But that is selling it a bit short. It can do a lot more.