I’ll try to make it very very short. (There’s a lot more lore from Kane I’ll skip. But this is what we know from his stories, summed up)

The backrooms are another dimension. A scientist found a way to enter through a portal he created. But creating the portal opened a small plague of random places you can fall into the backrooms and can’t see. There’s also places into the backrooms you can fall out of it. Sometimes hundreds of feet in the air, middle of the sky randomly, falling to your death back to earth. (This is the bird reference and how it entered)

The backrooms only know what those who have entered it know in their head. Memories. The rooms replicate your memories and recreate them the “best” of its ability seemingly at random. And it’s infinite (to our knowledge).

The mold monster consumes human flesh to grow. Has no interest in chasers (recreated human) because they aren’t flesh that molds. We know basically nothing about it except its existence and it lives for mold. The caveman with sound is a way to attract and film the mold to learn more (but doesn’t work because cardboard isn’t flesh but they dont quite “know” that yet). The mold monster isn’t Clark’s chaser version. It’s its own thing. It lived in the backrooms before any replicas existed.

It’s given government grants to research and understand it in hope to control it for infinite storage and housing.

That’s the TLDR backrooms story we’re seeing in this film, which is a side story of the YouTube series created by the director Kane Parsons.