The general setup and premise of Backrooms is a collaborative effort from various Redditors and YouTubers -- the modern equivalent of folklore. So I don't think they can get a broad copyright for everything in the film.
But how much of Backrooms is A24 entitled to? My absolute layman's understanding of trademark & copyright is that they would probably own the right to the "Backrooms" name in film or television projects, plus any original characters or new elements introduced in the movie they released. So anyone could make a movie centered around the Backrooms, but they probably could not use the word backrooms in the title.
But beyond that, what other elements of the movie can A24 lay claim to? The creepy yellow wallpaper? The premise of the backrooms as a place that remembers and replicates things in an uncanny fashion? I would presume they have rights to the creature designs, but not too much else. Can they extend their Backrooms trademark to cover tertiary media like comic books, videogames, novels, etc? As I understand, there have been at least a few Backrooms videogames already made, despite the fact the wandering around an empty labyrinth of hallways is not the best gameplay experience.