Yeah sorry, I wasn’t really using a true terminology. What I meant was, install planks of nice looking wood (stained/finished to your color choice) around that vertical surface in your pics. Personally, I would have the planks extend about 1/4-1/2 inch above the top of your floor (on 3 sides, but flush at the side where your stairs exit), and then install like a 1/2” (or whatever thickness gets you flush to the top of those vertical planks) by 1.5” or 2” with a nice finished edge to sort of “cap” over your flooring on the upper level. And on the stair exit side, you can pop on that metal piece that the other commenter posted.

Either way you’re going to need trim where the flooring ends and the stairwell begins, and for the trim to look right you’d either need a drywall crew OR go the route of a nice wood piece. That part is completely up to you: personally, I would get wood pieces to “box in” the stairwell - it’ll accent it/could looks a heck of a lot nicer than drywall there.

For what it’s worth, Sams club near me (Chicago area) has the Gerber partially hydrolyzed whey in stock

Thoughts on putting a beam in there instead of sheet rock? It’s change your look quite a bit, but depending on the span of the room it may look decently natural.