I was wondering what determines where a Corpsman is working (ie., Marine units, SEALS, Recon Forces, Seabee units). Like, is it a personal choice or where they assign you?

I'm no expert, but...

Mostly, it depends on your NEC (specialty). First, to work with SEALS, you need to be one. That includes corpsmen.

If you're 8404 (FMF), most likely you'll be with Marines or Seabees.

Other NECs will offer more choices. If you're 0000, you'll probably be in a hospital.

You usually have "some" say in where you go, but remember, Needs Of The Navy always rule.