The multiverse is hard to explain. Some use bells or boxes each filled with a universe. But within the zombie story, we get stuff like Agartha and Aether. That makes the multiverse harder to understand. But here is a try: The Wheel Spoke Theory, NabrZhunter once told me about this. I am not sure if this is like Treyarch meant it, but its a concept. Perhaps its true and perhaps not. Enjoy reading!
Dimensions
The word “dimension” itself has actually two meanings:
1. The way like you use it in 2Dimensional and 3Dimensional.
2. A universe in a multiverse, like the Original (Nacht-Buried) dimension and the Origins dimension.
With dimensions I mean the second meaning. So we know a multiverse holds many dimensions, realms, universes, or whatever you call them. In the Wheel Spoke Theory the multiverse is some sort of ball, or wheel. All the dimensions lay at the border of the big round multiverse, like the surface of a planet, lets say Earth. The number of these are infinite, like the number of trees on earths surface.
Agartha
In ancient stories, Agartha is located in the centre of Earth. In the Wheel Spoke theory, Agatha is in the centre of the multiverse. Just like the fourth dimension (first meaning) it is Impossible for our human brains to imagine this: Within a crust of 3 dimensional universes, a core: Agartha. But we can use an easy metaphore: Earths core. You can see this as a special universe, or dimension.
Aether
The aether is the interdimensional space. The space that is between the normal universes, but also between Agartha and the other universes. If normal universes are trees on the earthcrust, and Agartha is the Earthcore, Aether is the dirt and ground that separates the trees with each other and the core. One note, when Richthofen just discovered the Aether, he gives a speech about it (I believe in Eagles nest). He says the Aether is another dimension. Off course, he just discovered it, but if that is true, it is in opposite of in this theory, in which it is interdimensional space.
Gateways to Agartha
In the story, we have seen various gateways to Agartha: Shangri La, the Rift and the Crazy Place. They connect normal universes with Agartha. You can see these as tunnels, from the Earth surface to earth’s core.