The Undertale Multiverse is home to all Undertale AUs, Undertale, Out of Code Characters, The Void, The Anti-Void, etc.
It looks like a black box with multiple video cameras with several different timelines. Cords are placed all around the room.
It is a long hallway with doors to each universe.
It is a collection of rooms and hallways, some small, some big, all different colors, different designs, rooms full of trees and grass or rooms full of darkness and light, and doors leading to each AU. Other occasional things along the way (planted pots, boarded up windows, etc.).
It is a series of disconnected bubbles that are in constant motion. Bubbles sometimes cross over one another, causing crossovers between the worlds. Movement within this space can be done by any method, a 'floor' or 'staircase' appears where a character needs it though they could swim, fly or walk in any direction and orientation through this space even if they normally lack these abilities. Passing into the world itself could be done by opening a door, hatch or tearing open the fabric of the world. The door or hatch restricts the entry point in the world where the tear does not. Structures can exist within this place provided matter can be stolen from physical world in order to do so. Characters lacking a physical form gain them while in this space.
It's a large void filled with misty-looking versions of the AUs, showing what the current situation is like in each. If characters live in the Multiverse outside of their AUs, then their home seems to drift around their AUs like a moon orbiting a planet.
It's a large room full of floating papers for each AU. You can float to one, and tapping one makes it turn into a door. Going through the door makes a hallway of doors with signs of the areas, like Hotland, Waterfall, or The Surface. Going through those doors will send you to that spot in the Underground, or to the Surface exit where you get a happy ending.
A floating mass of islands, with random aspects from each AU.
It is a random rip in time that you can enter to go through to a random AU.
A big plaza with roads going off to each AU, branching out for their timelines.
A massive platform with some sort of hub in the middle. All AUs are connected via tunnels or wormholes, and each one has different accessible timelines, represented by bifurcations in the wormholes.
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The Alternate Multiverse theory
Essentially multiple multiverses of Undertale where events are similar in each one (such as Ink and Error existing in most), yet each is unique. Each multiverse could contain a different structure, such as all the ones listed above.
This idea is the only one considered 'canon' by the Undertale AU Wiki.