Generally a trainer will choose a jockey based on their familiarity with the jockey and his style, and the jockey's familiarity with the horse. You can add or take away anything from this question but the answer is " By the trainers" Familiarity, the jockey's uniform or what month he or she was born. But the answer is the trainer picks the jockey.
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Horses are chosen for the Melbourne Cup by the horse committee and the top 24 who can run the distance get chosen. They also needed to have one a few races they have entered. If they have not ran the distance before and they go through, it may be because the owner and trainer have thought the horse can run the distance.
The first leg of the triple crown the Kentucky Derby requires horses to 'qualify' by having graded stakes earnings. Then each horses winnings are calculated and the top 20 money earners are allowed into the race. The Preakness and Belmont are open races and do not require you to qualify.
The jockeys are picked by the trainer or the owner of the horse and its partly the race judges approval to!