For active duty they all go to the same primary flight school. Everyone gets to make a dream sheet. At the end they have “drop night” where the selections are giving out. The top of the class gets their picks first and it goes from there. Usually the fighters slots go first and then cargo and tankers. If you apply to be a pilot at a ANG or AF Res. Base you will fly what every plane that unit has.
Obviously, the first step is either earning your commission either active duty or through the guard/reserve and securing a pilot training slot. That is the most difficult part.
All AF student pilots initally fly the T-6 Texan II. That portion takes approximately 6 months. At the end of T-6s the students have what is called Track Select. That is when the decision is made to fly the T-38, T-1 or go on the Ft Rucker and fly Helos with the Army. The students get to rank the options, but ultimately the AF makes the decision. T-38s are the fighter/bomber track, although they can drop any fixed wing aircraft at graduation. T-1s is the the heavy track, and drop everything from Lear Jets to C-5s.
The students assignment to a specific aircraft comes down to their ranking in the graduating class and available aircraft that AFPC makes available.
I can get more indepth if needed. I just finished up a tour as a JSUPT instructor and a 20 year career as an AF pilot.