In “Death and the Planes”, I established a planar rubric that gave me The Prime (and as many elseworlds as I’d like) suffused and connected by The Grey, The Dreaming and The Abyss, all contained within the mechanisms of the Astral Temple.
Angels and Devils both come from these elseworlds, Demons from the Abyss, Undead from The Grey (and thence elseworlds and The Abyss), aberrations from beyond The Abyss, Fey from The Dreaming.
All very neat and orderly, but elementals proved a wrench in my works, in a way. Usually elementals are conjured from an infinite plane of their substance, or else the Elemental Chaos. Doesn’t work in my campaign, because there isn’t any of that.
I’ve still got The Abyss so I could claim elementals hail from there, but it doesn’t work for me. My Abyss is a structure of increasingly hostile impossibilities, and while “this is the land of sentient fire!” works for me, I can’t see summoning them being as hard as that sounds.
Instead, the Prime was once composed of Elementals, and now they slumber within it. The reason for the elemental character of The Abyss is its consumption of order: the laws which suppressed the elementals, stabilizing their substance and harmonizing their natures are removed in the unraveling towards oblivion.
Thus, when druids, fey, and wizards call an elemental, they are awakening an ancient slumbered already present in the material before them.
For this to work best, all material should be connected, and it surely seems to be. Spells like tree stride imply one should be able to fire stride or water stride, and as shadow monks show, shadow stride is already a thing. So that’s what’s going on when you summon an elemental: its animating spirit strides into the material.
What about genies, salamanders and gargoyles? They hail from worlds which are particularly infused with their chosen element. They are so infused-with and masters-of the stuff that summoning spells conjure them by sympathy.
Distances don’t matter; all fires are one and so any might call a salamander.
Where this falls apart is The Abyss. Substances unravel within The Abyss, that’s the point, and one cannot firewalk into that which is no loner true fire. That’s part of why intentionally summoning and controlling demons is so hard; their elements must transmute to affect The Prime correctly.