I’ll explain more about Dungeons & Dragons.
Dungeons & Dragons hosts a multiverse (the D&D multiverse) consisting of various realms and dimensions. Central to this are the Material Planes, which can be of infinite variety and host countless worlds like the Forgotten Realms/Kara-Tur, Greyhawk, Dragonlance, Eberron, Dark Sun/Athas, Ravenloft, Planescape, Spelljammer, Mystara, Birthright, Al-Qadim, Ghostwalk, Council of Wyrms, Nentir Vale, Exandria, Innistrad, Dominaria, Kaladesh, Zendikar, Amonkhet, Strixhaven, Ixalan, Ravnica, and Theros. Surrounding these are the Inner Planes, representing the elemental forces and consisting of both Elemental and Energy Planes, potentially infinite in their own variations.
The multiverse also includes the Outer Planes, which are realms shaped by moral and philosophical ideologies, potentially hosting an infinite number of alternate versions based on the beliefs and actions of their inhabitants.
Connecting these realms are the Transitive Planes such as the Ethereal and Astral Planes, which function as the glue of the multiverse, allowing travel and interaction between different realms and realities.
Additionally, there are countless Demiplanes, which are smaller, often bespoke universes created by powerful entities, each with its unique laws and constructs.
At the heart of this multiverse is Sigil, the City of Doors, acting as a central hub with doors leading to every corner of the multiverse, embodying the idea of 'worlds within worlds.'
The D&D multiverse is thus a sprawling cosmos with layers upon layers of reality, each plane or realm capable of containing its own universes, dimensions, and alternate timelines, governed by the myriad beings that inhabit them.