There were a crap ton of people cheering. Out on the bike course it was sparse except for one big up hill where the road was taken over by spectators (a bit dangerously as the road was not closed). There were other spots with spectators, though. People in chairs in their driveway and a number of people where the course crossed a popular bike trail.
The run was well spectated with only a few more remote areas without, at least while I was out there in the afternoon. I'm sure for the full distance athletes those remote areas were desolate and mentally taxing to not simply lie down and "embrace the sweet release of death" as ultrarunningmemes would say. The worst spot would have been about mile 21 of the marathon. Plenty of people would have been there at 10:30 PM after going in the water that morning at 7:00 AM.
The finish line was busy whenever I was around. I'd meant to be there at midnight because I've always heard for a full Ironman the crowd gets bigger toward the time cutoff despite most finishers being done. Supporters come out just to cheer for the random people fighting to the end to make the 17 hour cutoff.
When was your last race? I'm not sure anything will ever top how I felt in June after my first in-person race in 17 months. I didn't think about the pandemic for hours, and I spent all morning with a couple hundred like-minded strangers. I got home and laid on my floor completely blissed out.