I'm a ROTC/USU grad, and recovering flight surgeon. About half the class at USU is "prior service" - meaning some sort of miltiary afiliation. So prior enlisted, prior officer, ROTC, and academy types. The rest of the folks go thru some sort of fork and knife school - Officer Development School/ODS or Officer Impersonation School/OIS which is 2 to 6 weeks of how to be an officer. (See OIS comment). When I went to school (dinosaurs ruled the earth, there was only night and day, yada yada yada) there were like 33% Army, 30 ish% Navy, 30ish% AF, and 10ish PHS students at the start of first year. The graduating composition changed due to attrites, service transfers, etc. People switched for a variety of reasons like they really wanted to be in the <<fill in the blank>> but ended up something else, they married someone from another service, or they did (or did not) want to go straight thru to specialty doctor training. PHS numbers vary, and I *think* the USCG has a dedicated spot now. In terms of filling, the PHS and AF spots went fastest, then Army and Navy. There's no tuition, no costs for room/board/books, and some sort of O1 ish pay. (They keep playing with that, and I don't keep track). Years at USU don't count for pay or promotion, BUT are a nice 4 years added if you stick around for retirement.
HPSP is like a ROTC scholarship with 2/3/4 years paid for and 2/3/4 year committment. HSCP is probably more like the STA21 program.
There were a handful of prior aviators and "socializing" their plans with their commands varied. The A6 was going away, so there were a couple of those in my YG (I told you - I"M OLD). One guy had been a CAG paddles and another a TopGun instructor, so theoretcally still on the golden path. A couple ROTC/Trawing folks too, and they seemed to have more time to study for MCATS and such. I believe that their prior service time was credited toward rank and pay at half time served. For those who went back to aviation/flight surgery, I think their flight pay restarted where they left.
The service commitment does not include training and is 7 years. My buddy was a USNA grad and she owed 12 years exclusive of training. (5/USNA + 7/USU) I was a ROTC grad and owed 11 (4 + 7). We both were flight docs after our intern year and did doctor training afterward (both residency and fellowship, which I suppose is like going thru the FRS then TPS or TopGun). Once we each paid off our training debt, we were senior-ish O5s but OWED NO EDUCATIONAL DEBT. (Unless you're independently wealthy this is not inconsequential. The average medical student today will gradaute with $250k in school debt AND there are more US med students graduating from school than training spots). We've both since retired and are living relatively normal lives, doctor wise - found reasonably well paying jobs in our specialties in locations where we wanted to live, with employers that don't treat us poorly, or make me wear brown polyester or the current digital Garanimal suit.
The Enlisted to Medical Degree Preparatory Program (EMDP2) is fantastic - at least three of the Sailors or Marines I've worked with matriculated. It's a post-baccalaureate program / medical masters degree for enlisted folks that helps them with the pre-med classes/pre-clinical subjects , and take the MCAT. Like the USNA Foundation scholarships, it makes these folks competitive for any med school program, but most of them end up going to USU.
I spent a lot time in the fleet before doing my specialty training, which is a little unusual, but I"m glad I did so. I needed a break from academics and being a flight doc was really fun. Unlike most doctors, I worked closely with my patients every day and, on deployment, lived with them, which probably did wonders for my personality skills
Competition for flight surgery selection is like any other Navy job - depends on supply and demand. I had a friend who went to a civ medical school on an HPSP scholarship and went straight thru for orthopedic surgery. He did his first doctor job near some air station and figured out how much fun the flight docs were having. When his obligation was over, he told the Navy that he would stay for at least another tour IF he was allowed to go Pensacola and go thru flight surgery school, followed by some NAMI boondoggle. I had another friend who desparately wanted to be a flight doc but didn't meet the physical qualifications and ended up doing something on a ship. YMMV