Imagine an infinite collection of dreams brought on by an infinite amount of people. These are the planes. With enough belief, anything can form in the vast eldritch energies surrounding Elbion. Fragments of fleeting thoughts drift between shattered worlds as the few survivors of long forgotten expeditions cling to what humanity they have.
Every god, every alignment, and by extension every 'heaven' exists. When someone dies, their soul is carried into the plane they were most aligned with, unless they otherwise bargain away such a priveledge.
Few races, like the githyanki, know how to travel the planes. They do so, however, with great care. One wrong incantion, one wrong turn, will lead you into nothing short of the abyss.