Like, if you kick the ball down the field 50 yards, then possess the ball, the other team gets the ball there.
If you kick the ball down the field 15 yards, then possess the ball, you get the ball there.
What’s the rule that determines if it’s onside or not? Intent?
There’s no difference between those 2 cases - the kicking team can still recover the ball if it’s kicked down the field. The only rule on a kickoff is the ball must go at least 10 yards for the kicking team to recover it. Maybe you’re thinking of a punt? For a punt, the kicking team cannot recover it unless the receiving team has touched it first.