What or who determines what jockey rides on what horse?

Answer 1

The trainers actually approach the jockeys about riding their horses, but if a jockey is offered multiple mounts for the same race, the jockey chooses based on which one he thinks has the best chance of winning.

Answer 2

A jockey will work and ride for specific person and that person will choose out his/her horses. The best jockey will get chosen and then be chosen to ride the best racehorse.