What makes some people smarter?
Depends on what kind of "smart" you're talking about. There are lots of different kinds of "smartness." Some people are good at remembering things they see or hear, some people are good at being creative in things like drawing or dancing, some are good at understanding how things like cars and electronics work... the list could go on forever. No one knows for sure what makes some people better at some things than other people. So far, there is nothing for certain that is different between the brains of people who have high IQs and the brains of people who have average IQs. Geniuses don't have bigger brains or anything.
But, there are changes that happen in your brain when you are really good at something. For example, cab drivers in a big city have to know how to get around on a lot of complicated streets. They pretty much have to have a map in their minds of the entire city. It turns out that one part of the brain that helps you navigate, the hippocampus, is bigger in cab drivers! And, expert musicians have a larger area of their brain devoted to moving their fingers. But both of these things happen as a result of learning. So, the lesson is, anyone can be smart. Your brain changes to help you learn whatever information or skill you want to be smart at!