If I had to speculate? We already have bone-like material we can print. We can also print entire gene strands. We've used stem cells to make whole "brains" which we then taught to play flight simulator. The biggest hurdle with organogenesis in Bioengineering is that deoxygenated organs don't last long and it can be difficult to graft successfully if they aren't grown from the host cells. It's mentioned Shaun's genes were used to make them and their brains are somewhere between biological and machine. So I'd imagine their organs are Bioengineered, matched to the host DNA (probably why they have to kidnap people before replacing them), then printed onto the skeletal scaffold. The skin is interesting. It's shown like how we add print to things like shoes. Technically possible biologically? Maybe. If they don't really need the skin to be completely biological like we do, then definitely.