No idea on the holds, etc.

pick up your key at the front desk or at the automated check in kiosk.

online check out is optional. You do it before you leave the room, it offers a selection of checkout times starting with "now" in 1/2 hour increments. It's not dependent on online check-in. It shows you a summary of your charges up to that point. (ETA: I suppose you could do it after you leave the room too, as long as it's within your check-out time. Also, if you requested late checkout at check-in, the list of check out times will go up to that time.)

At US properties, it usually doesn't matter if you check out or not. If I don't check out I usually just drop the key packet at the front desk to let them know I've left.

The benefit of using online check out IMO is that it lets them know you've left and guarantees that you get the folio emailed to you and there's no need to deal with anybody at the front desk on the way out.

I usually don't bother with online check in, but I often use online check out.

-David