Each Domain reflects some aspect of the Plane of Mortality, both as a whole and in its parts. For example, Wyrlde is a part of the Ephemeral Dimension, and is reflected by Yrthe, Kismet, and Shades in different ways.
Likewise, Heaven, Hell, The Dreamscape and Nightmare are all reflections of the Fairywilde, as well, and this follows throughout.
Sometimes it helps to have it laid out simply, for understanding, but it always essential that one remain aware that all of them occupy the same are, just “out of phase” or “at a different frequency” from one another, like a bunch of marbles all in the same space at the same time.
This shows the relationships between the assorted dimensions, and thereby the relationships between beings like the miasmata, Fae, Angels, Devils, Shadows and Nightmares.
In terms of cooperative natures, Celestial/Radiant/Shadow and Infernal/Necrotic/Nether are the best known.
There are obvious oppositions in play Celestial vs Infernal, Radiant vs Necrotic, Shadow vs Nether. There are also less obvious oppositions: Shadow vs Necrotic, Radiant vs Nether, Radiant vs Shadow, Nether vs Necrotic.
There are conflicts of a tripartite nature beyond those: Celestial/Nether/Necrotic, Infernal/Shadow/Radiant. The relationships and hostilities are constant and involved and become issues that the Powers must deal with when engaging the service of Denizens.