They're usually aftermarket, mostly by marquipt going under the brand "tide rider" they make boarding ladders and stairs. Slight chance it may have been a factory option, but much more likely something added by a former owner. I doubt there is a "right" mounting point, although the pics above make sense for lining up with the ladder to the bridge deck.
When these boats were new this wasn't a problem yet, floating docks hadn't become popular. When I was a kid everything was wood or concrete docks and you never had a problem getting on/off unless you had a small boat then you had to scramble up. But since the 1990s everything has gone floating, and getting on and off a big boat is a hassle without these.
Fair warning get ready to bend over and put a stick in your mouth, these silly stairs are about $3k. Thank goodness mine came with the boat.