In it, the cosmos is divided and sorted into various polarized Planes, each describing some integral Idea (or more often, a Classical depiction of the afterlife shoehorned into an alignment) represented in physical form.
You go to the Plane of Fire and it's like 'Ah! There's fire everywhere!' And things behave like fire would. You go to the Nine Hells, and shit is just Evil all over. Go to the Positive Energy Plane, and everything is LIFE, vaguely.
Point is, they're physical places. You can go to Heaven and come back. You can book a vacation in Avernus with your local interplanar travel agency. They have places one can return to: the City of Brass, the dark mirror world of the Shadow Realm, the Fifth Layer of Hell, etc.
All of this I've described I'd like to put into contrast with what I'm about to describe: the...
Metaphysical Mathematical Cosmos!
It's, like... math and shit...
In the Metaphysical Cosmos, the Planes are not places one can go. They are places that you always are: either upon one side of the Plane, the other, or straddling somewhere upon the median. The Planes are, essentially, mathematical Planes.
|Like, this kind of Plane.|
Each Plane represents some ethereal Form. We're talking, like, the Theory of Forms here, Plato and shit. Each Plane is a "timeless, absolute, unchangeable idea" (quote source). Most importantly, each Plane is a dimorphic Idea - it must contain a state which is below the Plane, and one which is above. This differs from Plato, in which ideas are monomorphic.
Though they stretch infinitely, they do not occupy space. You can't go to these Planes and fly around and have Plane adventures on them, at least in the traditional sense (more on that later). Everything in existence, from gods to the last grain of sand, exists relative to all Planes simultaneously. There is a relationship between each Plane and every thing, and every thing exists in either four states for every Plane:
1. It exists on one side of the Plane
2. It exists on the other side of the Plane
3. It straddles the Plane
4. It is coplanar
In this universe, the overwhelming majority of things exist on either one or the other side of all Planes. Only the Divine or the Strange can straddle or be coplanar.
Depending on the Plane, this will mean different things for that particular object. For instance, let us compare two persons: a pauper and a King, as objects upon the Plane of Power, which describes whether or not something has true agency (in this hypothetical example). A pauper does not have much power, but a King does. They both, however, have some kind of power, and so the pauper lies quite close to the Plane, but the King lies far above it. Something without power, say, a rock, might lie below the Plane. If the pauper were to somehow lose all agency, say if he were Charmed, he could very well traverse the Plane, to join the rock. An unpleasant experience.
Ultimately, this is simply a way of explaining how magic and divinity works in a particular universe. It's all Vector Math.
|Pictured: Augury, perhaps?|
* This spell would rather block one from traversing.
These Planes being mathematical, there exist many combinations and relations: Planes may be parallel and never intersect with one another. Some Planes intersect with many, and their relationships may be described. Take a series of Planes: Power, Wealth, Faith.
A pious King, a Hermit Aesketic, and a Squirrel with a huge winter store would likely find themselves in 3 of the 8 different spaces within this conjunction of these three Planes. The pious King would have Power, Wealth, and Faith. The Hermit would have Faith, and maybe Wealth, but not Power. The Squirrel would have Wealth, but not Power and Faith.
So... Why bother with this? Why would it matter? You can't go there, right? So isn't this all a bunch of Ivory Tower nonsense? How it is game-able?
Astral Projection, my friend!
The kind of Astral Projecting, anyway, based on Forms. Cavegirl's Version.
The kind that exists to translate the metaphysical states to a physical understanding, where one can pluck at the ideal like how one plucks a violin.
Now, To Make Things More Complicated...
Let's add a layer of complexity to it all, by assuming that all object-vectors within existence are subject to Newtonian Physics. Everything bounces and collides with everything near it, all while simultaneously drawn towards each respective Plane by this gravity-like force, for any particular thing resulting in this kind of Brownian Motion - the chaos of existence.
But why, you ask?
Well, so the astral-surfing Monk can roundhouse kick the Lich into another metaphysical realm. KUH-POW!
Admittedly, this is a dumb idea. But its so up-its-own-ass I love it.
Here's how it works: Unless one is reasonably orthogonal to the respective Plane the monk wants to roundhouse kick the Lich across, the Lich is going to go sailing in pretty much any way imaginable.
So, for instance, the Monk wants to Astral Kick the Lich across the Plane of Power, rendering him powerless. The Lich, normally, has far more power than the Monk, and exists further above the Plane than the Monk does. If the monk wants to smash orthogonal to the Plane of Power, like a Newtonian pinball into the Lich, it'll only send the Lich higher above the Plane, making them even more powerful.
Forgive my terrible illustrations, please.
|This is the initial attempted Astral Kick.|
|And this is what we call: A Bad Thing.|
In order for the Monk to Astral Kick the Lich below any Plane, the Monk must be above the Lich in this Plane, not below. Which means, to kick the Lich below the Plane of Power, the Monk must have more power than the Lich. Not an easy feat, by any means. Doable, though.
However, there may be some Planes in which it's easier to get on top of the Lich, say, Fear, Empathy, Love.
There may be allies that the Monk would like to Astral Kick, like some kind of metaphysical yeet, in the effort of support. Make an ally more empathetic, more wise. To do this, the monk would need to be inferior to their peer in regard to the Plane in which they'd want their ally boosted.
So, to summarize: three functions for Astral Kick.
With regards to a particular object and to a particular Plane:
 = Plane M = Monk O = Object <-> = Velocity
1) If the Monk is closer to the Plane than the object, the Monk may kick the object further from the Plane.
 M -> O to  M O ->
2) If the Monk is further from the Plane, the Monk can kick the object closer to the Plane, or even across the Plane.
 O <- M to <- O  M OR  <- O M
3) If the Monk is on opposite sides of the Plane as the object, then the Monk can cross the Plane, while causing the object to travel further from the plane.
O  <- M to <- O M 
Complications/Failures of Astral Kick:
1. Aforementioned: make Object more of something than you wanted it to be.
2. Random Vector: send Object across different Plane than intended.
3. Accident: Hurtle oneself across unwanted Plane.
4. Collateral Damage: Exert astral force on other than intended Object, causing a bowling-pin effect of scattering multiple objects across multiple potential Planes. (In that case, related objects across space and time could very well cross unintended Planes.)
Okay... Enough of this insanity. I'm done. Whew.
P.S. (Of note: it's probably safest to ensure that these Planes are Non-Euclidian, or else they must all intersect or be parallel with each other, and that can be difficult to manage.)
P.P.S. (Also of note: This representation of reality allows a theory of magic and everything based on Vector Math. All things exist potentially not as objects, but as vectors. Magic is a way of transforming these vectors hard.)