Some go to college summer leagues, most play on a team of their choice.
I'll give you some anecdotal information on all of this...
* We had some discussion with our younger son's future college coach about this. His view was this is the last summer to be near home, have some fun with friends...enjoy baseball without a zillion evaluating eyes. He recommended he play for a competitive locally-based team.
* A former HS teammate of our younger son (now a freshman D1 pitcher) went to a summer college league this past summer...came back and didn't fully endorse it. He threw 13 innings all summer. 'HS kids aren't treated the same,' was his observation.
* Our older son, now in pro ball...formerly a college pitcher and a veteran of a couple different college summer leagues highly recommended AGAINST sending the younger one to a college summer league after senior season. Said he wouldn't be treated well by his teammates and would be somewhat of a misfit.
But then...there have been a number of local kids who did play college summer ball before college and it seems to have gone well.
Different perspectives, all valid. Our younger son will almost surely be staying local, playing for a good team with friends. In the Fall, he'll be off to college many miles away and with the schedule of a college baseball player (including summers) likely won't be home much at all over those 3-4 years.