Multiverse

The multiverse is the existence of many universes, dimensions, pockets, alternate realities that exist in the overall universe.

Background
Hell has conquered many worlds across the multiverse in its war to conquer all.

VEGA has monitored quantum abberations, in the fabric of the multiverse. He notices tenuous reality beginning to frament, signaling the dawn of a new age of chaos. The dimensional abnormablities and hyperfluxual phenonmena maybe signalling the multiverse is imploding on itself, where countless battles are fought between the Doom Slayer and demonkind.

VEGA has created an interdimenional man-machine gateway that allows the Slayer to engage in these skmirshes directly. As the Slayer enters the gateway, his surroundings take the form of an arena, in tandem with the arrival of adversaries ready for batt.e Seemingly extracted from the data of his experiences, the opponents the Slayer faces are familiar but now transformed by some unseen force.

With further analysis Vega observed ephemeral distorations within the fabric of reality; scrambled reflections of the physical world. As the slayer interacts with this anomoly, a peculiar ripple in the anomaly's composition emerges, seemingly reacting to the Slayer's presence reflexively. The anomoly reshapes itself, calibrating itself to the Slayer, responding to his memory, acestral, and memetic data, before ultimately projecting itself onto the Slayer himself.

Behind the scenes
This lore about 'multiverse' is in a section  of the Art of Doom Eternal pertaining to odds and ends like artwork for collectables, easter eggs, costumes, multiplayer (characters/arena maps), and barious skins for the Battlemode "transport Gate" (the background seen when changing appearances of the Slayer, his weapons or his enemies). Some future skins will apparenly be based on other ID characters and franchises.

This appears to be a lore based explanation and reference to a number of game mechanics and easter eggs. The first is it appears to be describing Battlemode/Multiplayer games, secondly it appears to be referencing the Doom Slayer having lived many lives(potentially explaining the Extra-life mechanics), it also explains Player Customization features in the games he's appeared in, it also explain 'easter eggs' and things he has laying about his office: collectables, costumes, gate changing feature, all of which he may have pulled from worlds across the 'multiverse'. It also hints that previous games he's appeared in are all versions of himself, or his own ancestors (B.J. Blazkowicz, for example) or versions of himself in alternate universes (ex. Flynn Taggart). Through quantum mechanics of multiverse, he is able to touch into other worlds he existed in, and remember their memories.