Can someone explain how a kindle works?

I’m curious cause I got one for my mom for Christmas cause lately she picked up a new hobby of reading and she was reading some book on her phone and she was saying she’s really annoyed cause it’s a really good book but each chapter costs money.

So, on a kindle is there like an App Store for buying books or something?? Is there any free ones? Are they regular price to normal books?? Do you make an account or something? Anyone who has one or knows anything pls let me know thnx