I will say this now and explain later, so people don't have to read it all to hear the theory. Also I will admit I'm a amateur at the Backrooms lore.
My theory is that the Backrooms are a fourth dimensional petri dish.
So other people probably have came up with this theory, but I have not seen it as I don't really browse the community. So I apologize if it seems like I stole your idea.
1 dimension or 1D = X-axis
2 dimension or 2D = X-axis and Y-axis
3 dimension or 3D = X-axis, Y-axis, and Z-axis
4 dimension or 4D = X-axis, Y-axis, Z-axis, and Time
This is a very basic explanation about dimensions and I'm no expert so I might get things off by a large margin.
A way to think about it is if the Tardis from Doctor Who was stripped of everything that makes it a third dimensional being/thing and wants to study third dimensional things.
Why fourth dimensional as time and space is bending in ways 3D just can't. Matter and energy seems to be created which breaks our 3D understanding of physics. Null zones are a tear in space and time, and once in the Backrooms you can be thrown forward or backwards in time or objects can be. So Highly Doubt aliens or anything from the third dimension can do this, as we don't even have a guess how much energy it would cost to do this. Then space is just moved causally like stuff stretching or shrinking, like parts of the dimensions that are pulled apart little to stretch.
The petri dish part is why the backrooms even exist in the first place by my theory. For imagine you work as a scientist that has found a living 2nd dimension universe. It was theorized, but living was never proven. So you want to experiment, but do to the difference in dimension you can't enter without destroying or causing its doom. So why not create a 2.1 dimension that is on a different layer, maybe a piece of paper that you layer over or underneath it. That way you can add or subtract layers without touching the 2nd dimension. So you run your experiments on this level most likely automated by computers. But then somehow actual living 2D beings enter your experiment. Of course your duty is to understand, so why not study how these beings live. You will need to set a point in time where you can't reach back due to paradox and instability, so why not when they first set off your alarms. Then you can connect the experiment layer with the real layer and slowly make copies with tracing in someway. You know the copies will be uncanny to the 2D people, but you can't do 100% accuracy as you ain't 2D. But wouldn't it be grand if you gave the 2D people away to enter and modify it by themselves. It becomes less uncanny, but something is still off it's still unnatural for 2D people. Now you can copy the work 2D people bring to the 2.1 dimension. That way it is more genuine and you might find out what makes them alive verse anything any other human made. As humans just make lifeless art. You still don't know how they tick so why not copy the most basic living thing that can be copied, and make it into a 2.1 dimension living thing. Maybe a Bacteria or something Micro that can be copied to all other experimental layers. It will stay in the 2.1 dimension but since it is slightly higher now in dimension energy it can penetrate anything 2D. It will take scans and slowly adapt like all life. So why not make this Micro life try to make 2D life even though it is 2.1D. It might make weird sounds or not understand 2D life, but better then anything 3D beings have made. It will act on animal instincts but it will stop killing hopefully. After all you didn't make it to kill; its just natural instincts of said living beings. This project is getting bigger and you can't track all 2D deaths. But all these successes slowly break down the barriers between the 2D and the 2.1D space you made. It should have a gap between dimensions but the 2D people are too curious to let this fissure close, and have the gap come back. So they made a permanent passageway. More and more 2D things and people slip through to the 2.1D which makes it more like there home dimension, but in the end still uncanny for it is 2.1D not 2D.
If you did not notice; this story is the Backrooms theory I have. Just change 2D and 2.1D to 3D and 3.1D. Then the 3D space to 4D. Also 4D thinking is not something I can do so I don't know if this is a single or multiple entity acting or if this is how it would go down. This was to just get out how I think about the Backrooms and what it it. And it seems like the backrooms has some intent just not direct; like a scientist doing experiments.
The Backrooms seem rather hostile to 3D beings, but not in a purposeful way. For the Backrooms bend space and time and so it could kill anything if need be. But this is a petri dish and explains why humans are allowed in. Like a petri dish it could be contained and nothing can get in, but a accident happens it lets things in and makes your work so much more meaningful or worse. It can makes your true goal so much closer by many different reasons. So why not let it ride. There will be other petri dishes that are still secured, but now there is one that is speeding all research ahead.
I'm posting this so I can get this out in of my mind, and I hope this will allow me better sleep. Also sorry for the grammar.