They’re not printing the metal parts. They’re printing the frame (as in “polymer framed handgun”).

The frame is the part that gets the serial number and is considered the controlled part of the gun. Rather than the trigger, the springs, the barrel, etc.

Other than the frame, which requires an FFL for transfer, especially across state lines, the rest of the parts can be ordered and shipped from anywhere and are not controlled.

Mind, that’s changing, again notably in CA, as they now talk about “gun pre-cursor” parts.

The 3D printed frames are similar to the “80% lowers” which are aluminum blocks that are “80%” complete AR-15 lowers (the lower receiver, again, the controlled part of an AR-15).

With straight forward machining and some jigs, those chunks of metal can be finished into an operational lower receiver, and the rest of the rifle can be assembled from disparate parts ordered from anywhere.

The original “ghost gun” before 3D printers enabled folks to assemble Glocks in their garage.