The Primal universe is the oldest and biggest universe known to exist. (If you're a human, that's the universe you're from.)
The Snarl is a little universe that failed to fully bud away from the Primal universe. It's separate, but still entangled at its edges, hence 'ensnarled'. Fiends live in this universe.
The Clashcradyne is the space, or void, between the Primal and the Snarl. It potentially extends to between other universes too. No-one lives here permanently.
The good stuff, baby! This wonderful universe is home to pulsars, comets, glittering nebulae, and of course humans. It's so huge that it isn't even possible to observe all of it.
The Snarl is an infinite plane, seething with pocket dimensions and areas of oddly curved space. It behaves like a toroid, in that travelling in one direction will eventually lead you to the other side of the universe. It is also possible to burrow down 'inside' the toroid and travel through the 'back ways' of intertwining pocket dimensions, and re-appear at another point on the 'surface' of the Snarl.
No-one calls the Clashcradyne home. The Clashcradyne is a space that's not quite one universe nor another. This makes it a dangerous place - all the dangers of the void of space, but with more reality-bending.
Nevertheless, there is a permanent human-built structure in the Clashcradyne. The Clashcradyne Research Station endeavours to find a way to use the Clashcradyne to fast-travel through the Primal, and to research magic and exotic materials that can be created only in this bizarre void.
The Primal does contain magic. It doesn't appear to contain any, because the Primal universe is vast. The magic in our universe is spread so thinly that it's unuseable for all practical purposes.* The Snarl is tiny enough that its magic is dense enough to exploit as an energy source, and the Snarl is therefore home to magical beings. These beings are collectively known as 'fiends'. Some of them are commonly called 'demons' or 'fae'.
*Humans actually do possess a nonzero amount of magic, and it is the magical anus smell. If you scrounge your fingers around in your buttcrack through your clothes, some of the smell will somehow transfer to your fingers, even through multiple layers of clothing. This is physically impossible, and the process is entirely magical.
Primal creatures (humans, spiders, geese, aliens, etc.) are those who inhabit the Primal universe. They are defined as being entirely physical: consciousness is electro-chemical and cannot be separated from the meat shell. Primals are unsouled and cease to exist once their meat shells are destroyed. Fiends regard humans with mild revulsion, because humans manage to be conscious despite being soulless pure meat.
Fiends are those beings and creatures who inhabit the Snarl, commonly called 'demons' or 'fae' by humans. 'Corporeal' fiends are defined as being both physical and spirit. Their consciousness is spirit-based, and therefore can persist without distortion even during near-destruction of the meat shell. Even if the physical shell is totally destroyed, fiends can usually find a way to regenerate.
'Incorporeal' fiends are defined as being entirely spirit. They have no permanent meat shell, and are therefore not alive (no metabolic processes). They do not have distinct 'selves', merging and splitting as required. To interact with physical objects, they must inhabit a temporary body made of matter.
Corporeal fiends can sometimes become incorporeal permanently by mistake. This is usually regarded as undesirable.
A corporeal fiend's meat that has been separated from its consciousness can't do much by itself, and will probably just fall apart after a few days. An incorporeal fiend might try to possess it, but it probably won't be very good at controlling it.
It is possible to access the Snarl from Earth, using the established pathways through the Clashcradyne. It could therefore be possible that the Snarl is accessible from other Primal planets too, which would mean that the Snarl can act as a gateway between points vast distances apart in the Primal.