Stupid theory: The Backrooms (in the first 9 levels) are different time eras glitched into existence and left in unfinished states, kind of like god beta testing time eras, this is mainly coming off of that the levels are like endless versions of the most boring common job you could have in a given time era. This doesn't really go anywhere its just a thought
Level 0 - 1990s dull office jobs
Level 1 - The boom in warehouse use because of online shopping, maybe 2010?
Level 2 - idfk maybe its a sewer kind of deal
Level 3 - huh
Level 4 - 2000's office I guess, its like a refurbished level 0
Level 5 - 1920s hotel worker, duh
Level 6 - The era before time, cold and empty and you shouldn't be alive here
Level 7 - I guess the ice age or some shit idfk. The ocean is the temp of liquid nitrogen and gigantism exists very exaggeratedly
Level 8 - bein' lost in a cave in the prehistoric era