i've tried looking this up, and everything just says to use the wall or hammer tools - but those don't work! the hammer deletes the entirety of my stairs, and the wall tool doesn't register the underside of the stairs as a wall to remove. i really want to use the stair cubby by ohbehave, but i can't figure out how to get it to be functional i see that there's open underneath stair cc that come without the walls, but is it cc only or is there a way in-game?

It's only possbile using Mods and CC which I've had installed for years because I ran into the exact same issue as you're having now.The CC you found is really the only way o fix the issue.It's what I did back when I was having your issue.

The underside of the stairs is part of the stairs model, so no, not possible without CC. The only item on the stairs you can to some degree modify ingame is the railings (they can be removed on some of the straight whole stairs, at least if I remember correctly).

The only open under stairs the game has are the glass set and the metal ones. You can't remove the back from stairs, its all one mesh. Even if you use those open stairs it may not work unless you also have the mod that lets under the stairs be used.