I imagine a foundry would use solar power and lasers to heat up the material. No atmosphere means less heat energy wasted. My thinking has been how to get enough actual build material to build something like an O'Neill cylinder. Well you'd need really thick metal plates. And then you'd want to get them into orbit without rockets. And these stations would likely be at the same orbit as Earth or nearby. Mainly because of how much sun energy you get around here. Going out to the outer solar system is a different beast all together.