I found an ARG in TikTok that looked pretty interesting, so I searched more about it, read a lot of things and ended up here. I thought I discovered about it in a Mr. Ferrante video, but this was shortly after.
from a youtube video about memes compilation, there was a minecraft video, in that video, a person was fighting a spider when he went through the cave wall and entered a huge yellow room, i went to look in the comments about what it was and I saw one that said it was from the backrooms, hence I became more interested in the backrooms and joined to r/backrooms, at that time the backrooms fandom was the official wiki used by r/backrooms.
I know the exact video your talking about I remember that from back when the backrooms was the biggest meme circa 2018. I remember when it was a shitpost to post an image of a smiler at the bottom of a stairway to level 1 and now thats just canon
I'm not really sure where it began to grow but it was 100% from a reddit repost of the original 4chan /x/ post of the level 0 image, the image and great description of level 0 lead to it becoming a meme which lead to r/thebackrooms getting made and from there people started making new levels (this pic is the first canon descriptions of levels 1 and 2, the level 2 picture is still the same on the wikidot which is pretty awesome). Really the backrooms just shitposted itself into existence which is fucking hilarious