It was previously thought that small asteroids could be placed into a large bag and baked to get out water and other materials. Experiments performed by Transastra show that this would take too long and need too much energy.
They have found what does work is optical mining. 16 kilowatt light sources focused onto small areas can efficiently produce outgasing in lab tests. They have designed a lot of systems and engineering for robotic asteroid mining.
They have designed a larger system that would fit into a future SpaceX BFR.
If asteroids can provide water and fuel it would reduce the cost of large scale operations in space by four times. NASA could spend $100 billion over two decades using near earth space resources to get the same done as $400 billion where near earth resources were not used.