I keep it simple.There is an infinitely wide (and flat) Prime Material Plane. It has three mirror plans - the Shadowfell, the Feywild, and the Ethereal Plane. Those planes constantly evolve to reflect the PMP in their own special ways. As the plane is infinitely huge, it allows for all sorts of campaign worlds to be set in it - and any type of PC to be played in any game.
There is an Astral Plane that is infinite in size and is the easiest way to connect the planes. My 'Spelljammers' sail this Astral Plane, allowing those with little magic to conduct business across the planes - which is lucrative. Traveling across the Astyral Sea to other gates that lead elsewhere in the Prime is also potentially a huge shortcut - something that make a lot of sense when you want to move a large amount of goods. Dwarves and Giff are the most common sailors - Dwarves have extensive mining operations in the Elemental Plane. Gith and Nezumi (Rat people - Pi Rats) are also common. My equivalent of Sigil is in the Astral Plane - a 'City' of multiple levels with a huge number of Gates, markets, and mysterious denizens. It is ruled by a Demi-God of Trickery.
There is an Elemental Plane that is filled with pockets of each element (and each paraelement). Dwarves are well known for traveling to this plane to work mines in deposits of Earth.
There is a Hell Plane. The 666 'layers' of the Abyss, the 9 Hells, etc... are all regions within this greater Hell. Asmodeus claims to rule the whole thing, but really he only controls the 9 Hells which cover a scant few billion square miles - only a small fraction of the mapped areas of the plane.
There is a Heaven Plane. It also is subdivided into various territories that align with traditional cosmologies. It is not unlimited in size. This is a reason why the Gods are selective about who is allowed to come here in the afterlife, and why many are reincarnated back to the PMP rather than enjoying an eternal blissful afterlife. Even those that are granted eternity may find ti revoked when other more worthy folks arrive...
There are Negative Energy and Positive Energy Planes. They're uninhabitable and a huge mystery.
Then there is the Far Realm - which is an alien cosmology that is slowly colliding into 'our' cosmology. The collision was first detected a few thousand years ago and has only slowly progressed in the time since discovery, but it is the source of Aberrations, Elder Gods and other alien concepts. The Far Realm is much older - and seems to be slowly collapsing.
Finally, there are pocket dimensions that connect to the Astral Sea. These may be as small as a box or seemingly infinitely large, but most of them are empty - until someone fills them. Their physics are not always like that of the PMP...
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