A large, spinning Cylinder with livable surface on the inside. Filled with atmosphere.
Rama ends up being an O'Neill Cylinder with a large interior surface, weather, and robotic maintenance crews. Homes and inhabitants are puzzlingly nowhere to be seen.
After humanity knows that Earth will be destroyed by aliens, they move to O'Niell Cylinders and other space stations in the shadow of Jupiter to avoid debris from the sun's destruction.
After humans recover data from the singularity of a black hole, they migrate off Earth into O'Neill Cylinders.