Asteroids present a rich source of materials and resources to use for the fabrication of space craft. These asteroids can be mined using Robotic Vehicles, and the ores gathered are processed at Automated Facilities.
It is not envisioned to return these resources to Earth.
The entire mining infrastructure is engineerable by the Robotic Vehicles, (ie, the system is self reproducing - almost). In other words, the Robotic Vehicles can also repair and maintain the infrastructure of which they are a part. The only part that is (potentially) not reproducable is the Central Processor Unit.
The basic idea is to launch a number of Robotic Vehicles with facilities into space together with a number of additional Central Processor Units and synthesise more Robotic Vehicles and Automated Facilities. Then together with the additional units, fabricate the facilities required for human habitation.
Without energy the Robotic Vehicles are useless. The energy required is electrical for functioning and kinetic for movement.