One day, I hope to actually play DnD.
unfortunately i am not aquainted with how the multiverse functions in any of those mentioned franchises with the exception of slight passing knowledge of the DC justice league episode where they get taken to an alternate dimension where flash died and they became the justice lords, but a throughline of multiverses is that they're primarily reflections of some form of the 'main' universe, 'the same world except [X thing happened] or [Y exists]' structureEasy peasy. I have written up several to be honest.
You could use 10-D Omniverse type; or something Tegmark, Brian Greene.
No, multiverse means multiple universes, whether parallels or alternate universes. It could also include dimensions on top of that.
DnD Cosmology is very similar to DC Comics, for instance.