After flying T6s (Phase 2 of UPT), students select Tracks ( e.g T38 (Fighter/Bomber), T1 (Heavies) etc) which determine the aircraft group they will specialize in. So to answer your question, yeah they somewhat have an idea, preferences are considered but from my knowledge it mostly depends on their class ranking upon graduation and aircraft availability, in other words, needs of Air Force, that ultimately determine what they will fly.
@geremi9656 After basic training they'll be graded and assigned to either fast jet or multi engine aircraft. So they have some sort of idea of what aircraft they get at drop night
Some of those were special mission aircraft, a couple of them were Air Guard sponsored, so their type was determined by what their ANG squadron flew. The others, not so much the airframe, but the duty station.
You're assigned to either a fast jet or multi engine; you put in your top three preferences but no guarantee what you get. Its based on current operational needs.