I've never bought into "multiverse" theory and never will. There's only one universe (the "uni" in there is a clue!) within which each plane is kind of a sub-universe.
All the worlds - all of 'em: Greyhawk, Toril, your homebrew, my homebrew, etc. - are out there somewhere, and if you've a good enough telescope you can see them from your backyard (well, as they were whenever the light left that is just now reaching your eye).
All it takes to achieve this idea is to assume fundamental physics vary a bit from place to place; and things work here maybe a bit differently than they do in some galaxy a billion light-years away.
The one in-game glitch in this theory is the unfortunate wording of the Planeshift spell (in all editions), which hard-codes the assumption that different PM worlds are also different planes. Garbage, I say; and I long since reworded the spell so it allows world-to-world travel within the PM as well as plane-to-plane travel.
Teleporting from one world to another, however, does not and cannot work if one applies the least bit of physics to it: when jumping from the planet you're on to elsewhere on that same planet, your speed of movement through space as the planet orbits its star and the star orbits its galaxy etc. is the same (or close enough) at your jumping-off point as it is at your destination. Thus, teleport on the same world works OK. But teleporting onto a different world moving at a different speed through space - yeah, splat. Even teleporting to the moon would be damn risky!
Planeshift, on the other hand, covers this problem off as part of its magic - kind of a divine blessing, let's say, for the traveller.
The other planes - all however many of 'em there are - are adjacent to this one and can be reached in numerous ways.