So, my theory is like that:
Almost all the entities are a real person who died in the backrooms
Let me explain if you didn't understand
There are the Frontrooms, and the Backrooms, the Backrooms probably was an workplace with phones, frames, the offices and something like that, but this workplace went bankrupt and they removed almost everything that was there, and then, the workplace was cursed by an spirit, and it was moved to another reality (thats why it looks so familiar)
After some years, some people have fallen here, then they starved (or died by another thing), and this spirit cursed them and then turned them into his "puppets", and now they're forced to kill people that enter the backrooms, possibly, the entities that are friendly escaped the curse of this spirit. And, i'm 100% sure that the game master and the beast of level 5 are connected with this spirit
Dollfaces are a different story, probably dollfaces was dolls that falled in the Backrooms and got cursed by the spirit, idk.
Partygoers may are childs that also fallen in the backrooms and got cursed, and they want to force people to turn into one of them.
I think my theory is stupid lol