It does have a lot of empty space. But I would presume it also has a lot of heavy metal structure to keep that space pressurized to a human-breathable pressure

It does have a lot of empty space. But I would presume it also has a lot of heavy metal structure to keep that space pressurized to a human-breathable pressure and allow entry and exit. Satellites don't need to keep much under pressure except maybe their maneuvering fuel in a small perfectly-shaped container with a single small hole.