I find it confusing as well which is why I ignore most of it and came up with my own cosmology. I have realms that are sort of the same as the Shadowfell and Feywild for example. On the other hand the ethereal plane isn't really a different plane of existence, it's just a place slightly out of sync with the real world.
So in other words, good luck trying to figure it out. It seems to vary by edition, publication and campaign setting. Your best source is the DMG chapter Creating a Multiverse or hope the new books like Monsters of the Multiverse gives more details.
In my mind, the Etherial and Phlogiston can be easily jived if one imagines the plogiston as energy and Ether is matter. . Both are in a constant flow state, but deep ether at its highest densities is comparatively inert. Phlogiston and ether are mutually exclusive, one displaces the other. Where phlogiston is predominate, It becomes flammable, to the point of explosive ness at its higher density. Phlogiston flows faster than ether, and dense phlog flows the fastest. Where ether is densest, it nearly solidifies to islands and asteroids, but still ephemeral from a long term perspective.
It also allows flame to exist without explosions, so civilizations can develop industry on a small level.
Where does phlog come from? The spheres. It is said that Spheres are warmer the older they are. This heat radiates into the ether as phlogiston and flows in the direction of sphere to sphere. So the spheres are bound to each other through a network of cosmic energy, much like a circulatory system in plants and animals.
I would posit that warmer, older spheres are capable of being farther apart from the network and other spheres. Younger cooler spheres far away would be very cold indeed, and any prime material within would tend to be cold, slow spacetime close to suspend ed animation, a la a frozen captain america.
Research needs to be conducted by Xanathar to compare sphere age/temp and the phlogiston flow speed.
And Im clueless about wildspace.
