I know the shape of aurbis and how most of it works except for one thing, where do the Aedra actually reside. I always thought they were usually in aetherius, however I then learned about the Aedric planets that help make up the system of mundus and that they're named after the eight divines.

Where do the Aedra actually reside?
What specifically are the Aedric planets?

The Aedric Plane(t)s are literally their bodies, we perceive them as orbs because we can't comprehend their true form or something along those lines.

So does that mean they reside in mundus?

More or less, yeah. They sacrificed a huge chunk of their power to Mundus, so they're bound to it. That's what separates them from the Daedra.

I understand that they sacrificed a large amount of their power to create mundus, but if they reside in mundus, who reigns over aetherius?

The et'Ada who didn't stay behind, and instead fled to Aetherius. The stars and sun are the holes caused by their exodus, they were literally like "NOPE, WE OUT" and ripped their way out to Aetherius.

The Aedra and Daedra aren't the only et'Ada ever, they're just the ones that didn't (or couldn't) leave.

I knew that Magnus and the et'Ada left, I guess I never really thought about where they went.

Wait those planets we see in artwork are actually the Divines? Damn, ths possibilities... What if Nirn itself is some sort of Divine? Or it was "given" to Talos? I love TES Lore, so much possibilities!

Nirn was created from parts of the Eight Divines and countless lesser spirits. When Lorkhan was killed during Convention, he was split into many smaller pieces. His body was torn in two and became the moons. His blood crystallized and became ebony. His mind left his body and became an entity known as the Void Ghost. His heart was buried in the soil of Nirn, which gave it its own divine spark. His soul was fragmented and doomed to wander Nirn, reincarnating over and over again in the bodies of mortals (known as Shezarrines).

The god Talos was born when three or more of Lorkhan's soul fragments (Hjalti Early-Beard/Tiber Septim, Ysmir Wulfharth, and Arctus Zurin) merged into one. So yes, in a sense Nirn is very much Talos' "own" planet.

Capital! Amazing story. Loving even more the lore now.