“Are you sure That we are awake? It seems to me That yet we sleep, we dream”
― William Shakespeare, A Midsummer Night's Dream
The Supernal Sea borders both the Mortal Realms and Otherworld with the lands of Annwn separating the Celestial Arcs and the Abyssal Arcs. These are the "northern" hemispheres of the concentric planar orbs. These rings radiate out from Trynnalta, but this may be purely mortal conceit. The upper most apex of the spheres is Unknown, the sweeping god-city and seat of divine power for the gods of the inner planes.
There may very well be other spheres out there. As time goes on, scholars and mages discover new planes and points in the cosmology with some regularity. While its true that Otherworld is its own separate plane, many speculate that since there's no mention of it connecting to anywhere but Annwn and Trynnalta that it too, separates the Mortal Realms from something. Many believe it to be Tír na nÓg, but until a concrete answer is found, Otherworld lingers as a sickeningly vibrant mirror image of the Mortal Realms.