So there were "buildings" on the outside.
...Which is a logical place for them, sort of, because the outside would have natural gravity pulling things "down". The Jenolan
clinging on to the outside like that is physically correct, then, too. As is the original crash.
Building on the inside would be trickier, because there would be no gravity other than that of the central star. The pull towards the shell would have to be artificially created, and then artificially damped at a distance so that pull from one side would not cancel out pull from the other. A trivial trick for even a Federation-level civilization, but the scale is vast. And it's pretty good engineering if the system still works even after just a few centuries of abandonment. Especially if it got its energy from the star, and the star now fluctuated.
Now here is a fun picture. Essentially, each pixel on the background is a continent if not a planet... It would take a bit more effort than Starfleet can muster to map even a fraction of this before things like "Starfleet" and "mapping" become outdated altogether.