Cosmology in the sense of the structure of the multiverse is how I've chosen to interpret Day Two.
Being a person of multiple faiths, my cosmology is sort of complicated. While in its way it's simple enough, and makes perfect sense in my brain, it's a bit hard to explain. My basic understanding is "shamanic" in nature. I believe in three primary worlds: Upperworld, where the Gods reside, Middleworld (I often refer to it as Midgard because of my Heathen influences, even when speaking from the standpoint of my Hedgecraft) and the Underworld (or Netherworld, depending on how whimsical I'm feeling).
There's a tendency to think of these as distinct layers, like in a cake. From time to time I fall into this trap and it's always to my detriment because that's not how things work. There's a lot more mixing going on. It's more like a trifle than a cake, with layers bleeding into one another, or mixed all in together. They are different and separate, but the Upperworld, for example, isn't always above. It might be right in front of you, or behind you, or below you.
You can bring these layers together. There are places and things (stone circles, for example, or barrow mounds) that act like weights on a sheet, like anchors, pulling two or more worlds together until they connect, overlap, exist together. These places are the same place; the stone circle is here but it is also there. It exists in two places simultaneously. I came to this idea through UPG but have seen it reflected in Heathen lore, in the sense that the dead were known to reside in a particular mountain, which would have been at the same time, and in the same place, both this world and existing in the next - at least in my interpretation.
There are other worlds. Some are small, some are bigger. There are at least seven and may be as many as eleven... but some are sort of attachments and could be said to be a part or element of one of the three main worlds. Depends how you look at it; I'm still not sure and may never be. The Norsemen named nine, and nine is a good number, although I don't think the nine in my head are the same nine as the ones they named. What of Alfheim, for example? Is it distinct? Is it part of the Upperworld... or the Netherworld? Neither? Both? Could be. That's the trifle I was talking about. The custard layer bleeding into both the jam and the sponge.
The Underworld is where the dead go. The dead of essentially everything. But there are other spirits there, too, that may not have been "living" in the sense we think of it here in Midgard, and therefore are not "dead". And here reside the more Chthonic deities. I tend to think of the various underworlds of mythology as different "countries" in this greater Underworld. So Hades has his halls, as does Hel. This isn't the only place that the dead go, but, in a sense, every place that the dead go has a bit of the Underworld about it.
The Upperworld, which I need to think of a better name for because "Upperworld" is lame, is where (most of) the gods live. I'm a bit of a panentheist in that I don't think of the gods as totally separate from this world, particularly if they're an "earthy" sort of deity. They are part of it and separate from it, at once. But Up There, so to speak (is it really "up"?), is where you'll find the walls of Asgard, and of Olympus, and of Unknown Kadath (ha), and all these places. Other things live here too. Spirits and guides, and so forth. And those the gods choose to take up with them.
As for Midgard. That's this place, but not just this place. That is, not simply what we can see and observe. If you Walk in this world, for example, you may see things that are of a different time, things that are almost metaphor, or suggestions that a particular place is "special" or "different". They are part of this world, rather than Above or Below, yet a different layer of this world. Not layer up or down, layer sideways, like layers of paint on a wall or pages of tissue over images in a book. Ah... I am having difficulty describing things today.
That's it, essentially. Three worlds that are more than three. Multiple worlds that condense down to just three, depending on how you think of them and how you draw the lines. My cosmology does its very best to defy human categorisation, much to my consternation.