The aliens act very much like humans. They have to constantly endure the stress of the knowledge that a large asteroid might hit them at any minute. Therefore, they also have to live in underground cities, far from the surface where they will have more protection from the asteroids. Since they have been living underground all their life, their sub-terrain cities have evolved from being simple underground shelters to complicated tunnels, cities, homes, just like earth, but underground. Since they have lived underground for so long, they have found many minerals in the ground, that they put to use.