What you have been told about the planes is wrong. Folktales, mysticism.
The truth is that the planes are like a tower. At the peak is the realm of absolute law and in the deepest dungeon absolute chaos. Good and evil are human projections of their own base desires. The pole of law radiates positive energy and the pole of chaos radiates negative energy and there is an endless current between the two poles. Picture a magnet. The human world sits roughly equidistant between the two poles. I say roughly because its exact position shifts and fluctuates with the energy currents. Sometimes it drifts "up" towards the pole of law and empires are built, great works done, and the people are oppressed. Sometimes it shifts "down" and empires crumble, wars are fought, and the people starve. The orientation of law as up and chaos as down is literal. While one cannot travel between the planes by merely traveling up or down the plane is shaped by its poles. Down, into the earth you find that the world is rotten, eaten by the forces of chaos. Up you find the ancient and terrible powers of law.
The planes are like a tower, but they are also like an ocean. Each level of the tower is distinct and drifting and on each level there are many islands, many realms, separate from each other. These places are what are often thought of as the varied material planes, and the outer planes. One can travel between these realms only by crossing through the emptiness between them. Unspeakable dangers await those that would step out into the currents. This place is what is commonly called the astral plane or the dreaming but its true conception is beyond imagining. The varied elemental planes are also mere aspects of this hinterland, with no true reality of their own.
Magic is done in the world by harnessing the energy of the poles. "Divine" magic is accomplished by direct tapping of one of the pole's energy (often with the assistance of an entity), "arcane" magic is the manipulation of the currents themselves. Both are dangerous and risk attracting the attention of the entities and old powers that dwell in other realms.
I started thinking about the planes because of the reimagining of Planescape that Scrap Princess is doing and because I am reading The Wizard by Gene Wolfe.
7 hours ago