How is a toilet flange installed on a concrete floor?

Use either a steel or brass flange. It is just the flange and doesn't have any pipe or connection under it. Place it over the pipe with the slots for the toilet mounting bolts in the right place and fasten it down with concrete screws. Preferred are the blue flat headed screws that don't need shields. Just drill the hole and screw in. Brand name of Tapcon or something like that.