Presented in Fizban's Treasury of Dragons and Tasha's Cauldron of Everything is the idea of the first world. An ancient world that was sundered by some great event, in turn creating the many worlds of the material plane. Also presented in Fizban's is the idea of dragonsight, a condition where dragons can psychically connect with alternate versions of themselves on other worlds.

This kinda confuses me. Does this mean that the worlds of the material plane are alternate realities? Or are they their own things, all contained within the material plane?

Quote from Bogdogs >>

Presented in Fizban's Treasury of Dragons and Tasha's Cauldron of Everything is the idea of the first world. An ancient world that was sundered by some great event, in turn creating the many worlds of the material plane. Also presented in Fizban's is the idea of dragonsight, a condition where dragons can psychically connect with alternate versions of themselves on other worlds.

This kinda confuses me. Does this mean that the worlds of the material plane are alternate realities? Or are they their own things, all contained within the material plane?

Fizban himself would likely pooh pooh a mortal need for specific truths and just say "Well, yeah" to the whole question regardless of multiple question marks presenting conflicting branches.  To a character, it's all uncertain, but if the character experiences enough they're likely or, liable maybe, to experience confirmations of both parts of your question.  If that's makes sense, it doesn't really, which is sort of the point.  But a dragon is the sort of entity that can fathom it, at least certain high minded ones.

I get that for most characters and campaigns in D&D it doesn't matter, but there must be some sort of true, defined explanation. Right?

Quote from Bogdogs >>

I get that for most characters and campaigns in D&D it doesn't matter, but there must be some sort of true, defined explanation. Right?

Fizban would likely say, "why?"

D&D is a game of smoke and mirrors, so why can't the lore be too?

Whenever my players get stuck in a wtf moment or are trying to pin down specific definitions for things that are just much bigger than them, I usually have a demon ventriloquize this, though I imagine the demon could be paraphrasing Fizban:

Rec for perhaps the greatest hour in television history

Some campaign worlds exist in the same material plane as others; this typically calling themselves the Prime Material Plane.  It is possible to travel between campaign worlds that exist in the same material plane via a ship called a Spelljammer.  Other campaign worlds exist in alternate material planes.  Some alternate material planes still exist withing the same cosmology; for example the world of Toril and the world of Oerth are both part of the great wheel cosmology.  These worlds can be reached via the Ethereal plane.  The same ethereal plane connencts to multiple material planes.  Each cosmology is as a bubble in something called the astral sea.  Some campaign worlds are in other bubbles, i.e. they are not part of the great wheel cosmology.  Krynn and Ebberon, and Kalimar are all different cosmologies than the one shared by Oerth and Toril.  I don't know exactly where Ravenloft is, but it is possible that ALL worlds connect to Ravenloft in some manner.  One would think that Ravenloft would then serve as the primary hub for worlds-spanning travelers, but Ravenloft is a "Prison" and once you find yourself there, it is very hard to ever leave it again. 

Quote from Fizban the Fabulous>>

This book insists on sorting dragons into little, understandable boxes as if the readers had only 100 years or so to live and their tiny baby-brains could only hold so much knowledge...

I would say that they where alternate realties  but they've grown up since then-its been 100000000000000000000000000000000000000000000000000000000000000000000 or so years since.