system
Given the following post, respond as if you're commenting on Reddit:
human
[DOOM classic] What's the deal with UAC's facilities? Why are they so confusing, mazelike and filled with secret doors?

I can assume that either the facilities are in ruin and the graphical limitations don't show it. Or maybe the demonic influence completely changed how the facility works. Or maybe they intentionally built it to be confusing as part of a ritual required to summon those demons.

But these are just theories.  
UAC facilities are confusing. They have doors hidden in walls revealing stuff, a weird zig-zagging walkway across a pond of nuclear waste, corridors that can only be accessed by walking through nuclear waste, doors with nothing but demons behind them and many more baffling design elements. These make perfect sense as gameplay mechanics, and make for exciting levels full of surprises and danger. But as an in-universe facility it's hard to grasp.  
Early shooters often took place in science facilities, but Half-Life presents a mostly (MOSTLY) believable facility where most dangers come from the place slowly falling apart, or simply having to access areas that scientists aren't supposed to access because they're dangerous.

But the UAC facilities don't seem to have much of a point. There's tons of dead ends, pools of radioactive waste and weird walkways.  
It's like the Winchester Mystery House in space.
gpt
It's actually a fusion of a few things:

1) Sustaining a portal to hell requires manipulation of sacred geometries. The layouts of these stations are intentionally designed to incorporate certain patterns and figures to facilitate demon summoning.

2) Hell has a corruptive effect on the world around it. We can see this in the alien vistas and mutative effects the portal has on living beings near it. When a portal is forced open, the world around it is shaped, terraformed if you will, to facilitate a stable portal and supportive environment for demons.