I've watched a few streamers play this
It has your typical gacha mechanics (pull different quality horse girls and support cards). Between races, you have limited resources to train the horse girl in things like speed, power, stamina, etc. Winning races and placement also gets you points you can use for different skills, such as a skill that makes you recover stamina on bends in the track, or acceleration boosts on straightaways. You basically play each horse girl through a career and there is a "breeding" mechanic where 2 of the horse girls pass on certain attributes to a different horse girl.
Apparently it was huge in Japan and has been out for a few years already as a mobile game.