Serious questions - what are these prep schools? Are they just private schools with fancy basketball programs? Who pays the tuition? Are the same as those "prep schools" for students after they graduate high school but before university? Asking for a friend.
It depends. Some are long-standing private schools that have developed national-profile basketball programs over the years. Some are boarding schools, some not; some religious-affiliated, some not. Generally, they have very good academics, and the athletes are expected to perform at levels similar to all other students. Monteverde, Brewster and Sierra Canyon come to mind. These schools have good alumni/donor networks, with a lot of kids getting scholarships (not just athletes), particularly those from less-advantaged backgrounds with particular talents.
Others are pure basketball factories that have limited facilities (access to a gym and weight room), and only a couple dozen student-athletes enrolled. The organization typically send kids to a local parochial school or online school a few hours a day for what seems like a minimum-requirements education. Kids typically don't pay much (or anything) for the experience, which is "sponsored" by shoe companies and "donations." Findlay Prep (closed), Hillcrest Prep, Huntington Prep and Prolific Prep come to mind. If you are getting the sense that I think these outfits are all kind of sketchy, you're right.
IMG is a unique animal unto itself, built from the ground up as an athlete-focused school, that has relatively decent academics coupled with amazing facilities. Some kids pay, but the most promising get scholarships. Again, you can guess where the money comes from.
And yes, quite a few of these schools will accept kids for a "post-graduate" year between H.S. and college. Kids who are "late bloomers," rehabbing from serious injury, or who need a year to repair an academic issue, are the ones who typically go this route. It worked for a friend's kid who graduated from a large North Suburban school, did a year at a New England Prep, and now is on the squad at Harvard.