This should probably be either a blog or a sandbox, but it'll get more attention as a post.
Big Ideas
The Backrooms are not neatly divided into levels. Levels are closer to "biomes" than completely different dimensions. It is not always clear where one level ends and another begins, they all merge into eachother. The same applies with domains.
The backrooms is incredibly vast. There are some areas that are populated by large civilizations, but because of its vastness, it is still normal for wanderers to be completely isolated for years.
People who enter into the backrooms do not necessarily come from the same universe. Many come from alternate timelines.
Domains
Primary Domain: contains main nine levels and some others
Siderooms Domain: Contains the Maintenance Level, the Hive, and various levels that are "behind the scenes" of the backrooms. The hive is there because many entities apparently originate from it. The Maintenance Level is there because it is supposedly related to the functioning of the backrooms.
Negative Domain: Mostly contains levels that appear to be "glitched".
Abilities
Manifestation: the ability to "spawn" things into existence through sheer force of will.
Noclipping: the ability to pass through a solid object.
Navigating: the ability to be able to navigate through the non-euclidean and constantly changing geometry of the backrooms.