A lot of professionals co own the horse or are catch riders that show Grand Prix horses to put miles on them so they are easier to sell. They are also sponsored by brands so they get tack and other stuff for free. Also a lot of professional riders are either really rich, come from horsey families, or both.( some riders are an exception to this like Kent Farrignton)
My trainers daughter, who rides professionally, gets the majority of her money from winning big classes according to my trainer. She also gets some from training her students and training other people’s horses. I think with the combination of training and the big classes they do pretty good. From one grand prix that she won, she won $7,700. The bigger classes you do, the more you can win money wise.