Correct me if I have anything wrong: Ravnica is a single large sphereworld, orbited by a Sun and Moon, within a chrystal sphere, stars being affixed to the inside of the sphere (pretty much as per Spelljammer cosmology).
Ravnica has two moons and it has it's own plane where Ghosts go, nicked named the Ghost District, although it's at least as big as Ravnica. It was sealed away by the first guild pact from the rest of the MtG multiverse for 10,000 years.
Honestly I am super curious how they fit Ravnica into the D&D universe, because a some spells and magic and other stuff interacts with the D&D, stuff like Banish, Etherealness, Planeshift, Demiplane, Conjure X spells, Summon Lesser Demon, Summon Greater Demon, Planar Ally, Augur, Contact Other Plane, Commune, Horizon Walker, Fiend Pact Warlock, Divine Soul (in some cases), Genasi, Aasimar, Tieflings, Shadar Kai, Eladarin, Gith, Teleportation Circle (can move you between Crystal Sphere worlds with the right code), Mordikein's Magifiscent mansion (creates temp demiplane), and so on.
In fact it would almost be easier to make the D&D Ravnica separate from the MtG Ravnica.