Separate reply for Lore:
The Shell is a world roughly 10x earth diameter, roughly 3x earth mass, and completely hollow. The shell itself is about 250km thick on average. Most of the external surface is glassy, endless plains with a wispy atmosphere.
Air settles into the valleys and craters, some of which are the size of earth continents. At about 180km below the plains, whether in valleys or Brobdingnagian cave networks, gravity is about 1G. Continue down and gravity reduces to zero once you reach the level of the interior surface. Go up to the plains and you have to deal with about 3G. Reaching orbit is possible, but with no atmosphere worth mentioning in which to aerobrake, few have returned alive.
The Shell is considered artificial by most scientifically advanced cultures, of which there are at least 20, with a war raging somewhere most of the time. Inside the shell, there is air. Breathable. Lights flicker and glow in the weightless, humid depths. Who knows what goes there.
How did humans get there? Who knows. An ancient colony ship, perhaps lost. Or were they created by the planet itself, which appears to be alive and occasionally non-eucludian? That depends on your religion.