As you can see from this post which has a graphic of all the colleges where each Sweet 16 team's starting five started:
1) A lot, if not the majority, recruit talent out of hs and develop them. Izzo loves doing this!!!
2) Two guys started out at UCLA but only Traitor Mara came to mind. But then I realized it was Berke at Nebby
Sweet 16 Starting 5 - College where they began their bball career