If I saw that shit in a game I’d immediately assume they’re being satirical or cringe on purpose.

Nope, not kidding. the receiver is the component that is registered as a firearm, and the rest of the parts that go in it are unregulated. That’s also why you may have heard of “80%” receivers. They’re not machined all of the way so they are not considered firearms and the purchaser can then finish the machining and install the parts.

In the United States, only the lower is considered a gun. There’s no check needed to buy the barrel or slide components, meaning you can print the lower, preferably from a fiber reinforced material, and just slap the unregulated upper components into it.