Do they replace fan belts.

Yes

Yes, they do, as well as a number of other mechanical things, too. I bet they'd be happy to tell you more if you give them a call.

If your getting tire work done, they might.... never hurts to ask, but if that's what you need look for a full service garage.

I believe they only do tires

I'm sorry, I do not know about mechanical work. If I had to guess I'd say no.