What job in the medical field is mostly about bones and why is that job interesting?

There is no "one" part of the medical field that is about bones, there are many. Why? Because bones are mechanical component of what holds the body together and allows it to manipulate things outside itself. Sports Kinesiology studies how bones and muscles work together to perform athletic feats and simply to move around. Study of Osteopathy is how the body maintains efficient movement and how to restore that in case of malfunction -- disease or injury. Surgery for restoring damaged or malformed bones and joints is a large part of medicine. Specifically, spinal surgery is about bones and of course, the spine itself, which is protected by and supported by the vertebrae.

If you ask me, the mechanical aspects of the body -- muscles and bones -- are fascinating because of the apparently ingenious ways such a seemingly trivial set of structures combines to do such amazing things. If you take essentially a piece of meat and a piece of rock (bone), and have it lift hundreds of times its own weight and then rebuild itself from the resultant stress, I'd call that pretty nigh a miracle.

So that's why those jobs are interesting.

An Orthopedist is a medical doctor who works with bones and muscles. This link might help you - https://www.medicinenet.com/script/main/art.asp?articlekey=4669