Former swimmer here. Most swimmers I knew didn’t even have bad shoulders, they all had bad knees lol
I can pop my left shoulder out naturally and was able to do so at a young age. I only went to PT for it (and my back) last year.
Jeff Cavalier (ATHLEAN-X) on YouTube talks about external rotation a lot to the point where it’s become a meme in the comments on all his videos now lol. The guy is a physical therapist and trainer, so he really knows what’s up. The stuff he talks about is exactly the stuff they had me do at PT. They’re honestly all the same when you get down to it but yeah.
The best one for the rear delts that I can tell you are a modified version of facepulls. This is the one Jeff likes the most, and what every physical therapist has you do for rear delt/rotator cuff work. It’s like a regular facepull, except that instead of your hands finishing near your ears, they finish more outwards. Your elbows should not be bent onwards. Basically, it’s a regular facepull except that you elbow is at a 90 degree angle instead of say a 30 degree angle like in a normal one. Not sure if that’s making sense. It’s very similar to the W in YTWL’s (arms should be higher up tho).
Speaking of which, YTWLs are a great way to bulletproof your shoulders. I don’t do all of them but they're still very good. The reverse cable crossover one (the Y) is great imo because it works your lower traps as well which is something that most people ignore. Doing the regular T part is fine, but I like Y better.
The last one I do doesn’t realy have a name that I’m aware of lol. I use a cable where I angle myself ~45 degrees away from the machine (so say I’m doing my right arm—I angle myself ~45 degrees to the right) and do a raise. It’s not a sideways raise, it’s in the opposite direction of the pulley. So it’s not quite a raise, but it’s also not quite a straight-arm pulldown either. But yeah, that’s what I do.
Bulletproof your shoulders.