There's a whole community for it! There's a YouTube channel called "Print, Shoot, Repeat" that actually talks about the gun police showed

What if I told you about “80% receivers” and parts kits that have been in use for decades by hobbyists?

• Individuals who make their own firearms may use a 3D printing process or any other process, as long as the firearm is “detectable” as defined in the Gun Control Act. You do not have to add a serial number or register the [privately made firearm] if you are not engaged in the business of making firearms for livelihood or profit.

“80% receivers” are a weird line in the sand to pick between “random hunk of metal/plastic” and “yeah that’s a gun bro” but words have meaning that (still) have to be defined in law, and you can build a 100%* factory looking gun with zero 3D printed parts and no serial number.

*Depends on your skill with tools and machinery ofc, but can be done with a hand drill and a basic file with enough patience