Are all toilet bolts installed from under the floor?
A typical toilet has a flange that's bolted to the floor and stays there.

Bolts are then turn upside down, with the head down, and the toilet sits on these. The threaded end faces up and the nut goes onto it.

So, a typical toilet install is closet bolts that have threads facing up. No need to go under the floor, be it a 3rd floor bathroom, a 1st floor bathroom or a basement bathroom.