Outer Rim is the mirrored version of the vanilla world. It's a fading memory formed by the one-mortal benefactors who've since moved on, and as they do, the world is enveloped in dust and fades back into nothing
The only parts of OTR we see are the parts that people would have lived in died in. Depths, Ancient Urban, Shaded Citadel, The Iterators
Motifs of all these things show up, but nothing of like... Sky Islands, or Farm Arrays. Because not many would have lived there, and so there would hardly be any surviving memories
The drills are upside down
The Starcatchers sit atop cities, as opposed to the cities sitting on top of the Iterators
2 megastructures sit next to each other, one of them rotted (clearly FP and Moon)
Spires meant to catch stars reach into the skies, compared to the filtration systems that go below
The world sits atop the edge of the firmament among all the stars
I don't think its LITERALLY across the boundary, but its a version of the vanilla world likely formed in the cerebral realms/ripplespace by the ancients, benefactors, echoes and the like