We learn from Ed Harris that initially, the hosts were androids - mechanical robots made to look like humans. This is also seen in some flashback sequences, and Robert, the kid who opens his face to reveal the circuitry inside. In episode 6, we learn that the hosts were later switched to the current 3D-printing process, in order to appear even more human.
However, what exactly is their physiology? Their energy source? Do they need food or store it inside a stomach chamber? Or are they charged overnight, since obviously they do need a source of electrical power for remote control and data transmission?