In college football, is the ball considered live or dead if it hasn’t hit the ground but bounces off the helmet of a downed player?
Is this in the context of a forward pass, or just a loose ball? Either way it shouldn't matter (and for a loose ball, even hitting the ground shouldn't matter). The ball should remain live.
Per the NCAA 2022 rule book, conditions where the ball becomes dead is covered by Rule 4, Section 1, Article 3, "Ball Declared Dead". That contains 18 separate points that cause a dead ball to occur. None of them relate to touching a player (except that item "a" (out of bounds), is later defined to be possible as touching an out-of-bounds player).
All the other dead ball scenarios involving a player refer only to the ball carrier. Touching some other player does not create a dead ball.
If this is a forward pass, touching a player but not the ground does not make it incomplete (item "i". See also 7-3-7-a for rules of an incomplete pass).
If this is a fumble, then normally the ball must be recovered by a player or go out of bounds before becoming dead.