3D printed guns do not “get past metal detectors.” This is not nuance. It’s not a matter of techicalities. You just told a bold faced lie, and you did it speficially to try to fear monger.
Printed guns still require metal springs, metal pins, metal bolts, metal barrels, and metal ammunition.

The US it is federally legal to manufacture your own firearm. There are some state level restrictions depending on which one of those you’re in. In the rest of the world it’s probably considerably more regulated, but then again in the rest of the world it’ll be tougher to get your hands on the all the other parts you’d need as well.