The rocks at the surface of the Moon are richer in metals than the crust of the Earth. They are especially richer in iron and titanium.

Without oxidizing air, it is easier to extract metals from the Moon rocks.

There is little doubt that it would be possible to build big spaceships on the Moon.

However, what is missing on the Moon is fuel. For interplanetary spacecraft, nuclear reactors would be preferable anyway, which could be assembled there from parts shipped from Earth, but for propulsion those still need a large amount of some working gas,to be heated and ejected.

It remains to be seen if there is any useful amount of water at the poles, but I doubt that there is enough for a long term exploitation.

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.