Or else behind the scenes, trillions of giant space hamsters running on giant wheels power everything and give the planes physical form.I saw this asked on another forum and the answer is the same, they are physical places, thought and idea is just what originally gave them form.
Not really in my opinion.DND cosmology has always been a complete mess. Just redo it to fit your campaign and move on.
Whatever way you want.how do the outer planes work? if they are realms of pure thought and ideas, how come they work like physical places?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.