It is a grandiose Creation. It consists of six planes of existences, half of them being indescribable and the other half being describable in mortal perspective. To illustrate, here is the official map of the Multiverse that came out nearly a decade ago.
Much of what is in the map is outdated, but it serves as a blueprint for improvising and creating new structures.
In the deepest level of the material world, there is the Microverse. Below Ryan Choi talks about it briefly—the foundation of everything. However, this would be the lowest level in the Light Multiverse. The deepest level would be the Dark Multiverse
However, this would be the lowest level in the Light Multiverse. The deepest level would be the Dark Multiverse, which is a subconscious realm that the main Light Multiverse floats.
As for what the Dark Multiverse contains, there are infinite nightmare worlds.
In order to understand how the worlds in the Dark Multiverse float, it is not just a reflection of the universes in the Bleed. Rather, they are, in a way cosmologies, albeit it depends on the dark universe. You would have to read the various stories from Tales of the Dark Multiverse and see why it is that way.
There is a structure called the Orrery of Worlds that used to contain 52 realities, although there had always been infinite realities even all the way before and after DC Rebirth. But, it has made it conspicuous that there are infinite universes again since Infinite Frontier.
Not only that, there are infinite multiverses. All of that is contained in the Bleed, a pandimensional barrier containing infinite higher spatial dimensions blocking the abstract realms.
There are also infinite number of timelines.
Every decision spawns a new universe
Now, this one will be confusing but there is a structure called Hypertime. What we usually see in the stories contains every universe and timeline in the solid world. They act like backup files when one is erased, like the time the Anti-Monitor destroyed infinite alternate realities with five being plucked out in the nick of time. Additionally, it depends on the Multiverse.
The other variant of Hypertime is meta. It is based on all of DC’s History from our perspective, not an in-universe perspective. Grant Morrison elaborates it more. Not too long ago, the recent guidebook for the DC Multiverse describes it as an Omniversal vault.
The Material World is pretty ginormous. No, Infinite!
The next ones are the immaterial realms. The lowest of them would be the Elemental Realms, those being the Green, the Red, the Grey, the Clear, the Rot, the White, the Corona, etc. I cannot be sure if there was been a retcon where the Realm of the Parliaments are in the same plane of existence as the Sphere of the Gods or a sub-level. This is the only information to suffice. These realms being beyond time and space make them incalculable as they are beyond spatial attributes.
After that is the Sphere of the Gods, which is the realm made out of possibility, belief, magic and the ultimate level of a concept.
The realms in the Sphere of the Gods are beyond description, and function on narrative and metaphysics. All realms in the Sphere of the Gods are beyond time, space and scale, meaning that it is meaningless to ask how large it is. One example would be Hell.
Planes of existence containing Limbo, the Monitor Sphere and the 5th Dimension would share the same values and descriptions like the Sphere of the Gods, only that it is beyond and more profound.
Next is the 6th Dimension, the highest level of existence that is literally the realm of impossibility. It is the control room where the Hands, Judges and other Overseers monitor the progress of the Multiverse. It dwarfs the rest of Creation
Keep in mind that the DC Multiverse is just one out of an infinite number of Multiverses floating in the Greater Omniverse. It is likely that those Multiverses beyond share the same characteristics.