Very few parts of guns require high-end machining and materials (barrels, specifically, are difficult/expensive). These are also not particularly highly controlled -- the barrel for a rifle can easily be repurposed as the barrel for a machine gun.

(A badly made barrel could be totally usable for a combat weapon; it just wouldn't last for 100 years and put a bullet into a palm sized group at 500m)

Receivers are historically the one part of a firearm which are considered to be "the firearm", and are not subject to particularly high stresses, especially on the AR-15 design. They can be machined by relatively incompetent people from aluminum, or made from plastic (Cav Arms sold these commercially for a while).

Guns just aren't that hard to make. The hardest parts of firearms to produce clandestinely are powder and primers, since that's more a chemistry problem than anything else. I'm not sure what the state of the art is in homemade smokeless powder, but black powder would be possible to make on your own (if a bit dangerous).