Kindly provide a source for this please.
There is none. I probably should've endeavored harder to convey that its just interpretation, but thought that most of it was pretty obviously interpreted. I don't particularly want to talk at length about why I think this way, but if you want it, I'll give it.
So where does poison and magic fit into this? The only time poison wasn't put in was in OB, and what would magic be?
Actually, poison evolved. It got shifted into alchemy where it more fittingly belongs. Notice how you've still got a weakness to (and I also think resistance to) poison spell in the game. Instead of being able to chuck balls of generic poison at people, it now actually makes sense. I suppose if you're asking what element is poison put into, then from oblivion's standards (where we actually get the whole elemental thing clarified and defined) I'd put it into earth. From Morrowind's and, to a lesser extent, Daggerfall's standards, I might go so far as to put summoned poison into the category of necromancy's contribution to destruction. But that may be bias.
As for what magic actually is? We know it flows from Aetherius. The Ayleids align it with their Light element, but I'd go so far as to say that Magic is simply energy. Using magic is directing energy to change or summon something. That belief of mine is evident when I describe how Fire, Frost, and Shock are different varieties of the same kind of energy.