Correct “Gun parts” are anything from triggers to complete uppers. Some guns are different than others but for the sake of this conversation, the only part on a gun that legally defines it as a gun is usually the lower. When purchasing a lower you need a background check (unless it’s an 80% (a lower without the holes drilled out)), any other part (triggers, barrels, uppers) do not require background checks.

In PA, you are legally allowed to build your own firearm (which includes 3D printing a lower as long as you’re not a felon) and are not required to register it anywhere.

The issue specifically in this article was the guy was ordering uppers and printing lowers.

(Also, not really giving out legal advice but to cover my ass I’m not a lawyer. This is what I know from my personal experience/research)