Did they ever actually tie the D&D settings together officially? THat just sounds like a terrible idea. Something like the Forgotten Realms could slot right into one of the bigger cosmologies but how do you reconcile Planescape with Spelljammer? Those both scream "top level cosmology".
Essentially, accorting to PS+Spelljammer unification, all the worlds visited in Spelljammer are part of the Material Plane, or Material Planes if you prefer. Different worlds, which I will note typically can't be accessed by 1:1 planeshift, exist in the Material separated by Wildspace. Spelljammers typically can't reach the Outer or Inner planes.
This was established back in 2e when the Planes were slightly better thought out. The Ethereal was the transitive plane between the Material and "Inner" planes, while the Astral was the path between the Material and Outer. Going from outer to outer or inner to inner or (in the case of Spelljamming) material to alternate material isn't accomplished by the same mechanisms, needing other pathways. For instance, if you want to get from the Abyss to the Nine Hells, you can boat down the River Styx. Want to go from Great Dismal Delve in the Plane of Earth to the City of Brass on the Plane of Fire? Find the right guide and you could cross through the paraelemental plane of Magma. Need to get from Toril to Oerth? Hop a Spelljammer. Want to get from Celestia to the Plane of Water? You won't be taking a "direct flight" since there is no (known and usable by mortals) Transitive plane between the Outer and Inner (The Oridal Plane is, in setting, theoretical and mysterious)
Or you could visit Sigil, City of Doors, which is unique in that it could have portals to literally anywhere, hence why it's considered the Center of the Multiverse.