They actually are open source, and there is indeed a lot of material out there to help people with everything they need to build their own. The only part that is illegal to make on your own is the part with a serial number on it, for tracking purposes.

The only part that is illegal to make on your own is the part with a serial number on it

That’s 100% incorrect (by federal law, can’t speak to any specific states) and is most often the only part that is printed, for exactly that reason.

Technically, you’re supposed to give it a serial number and register it with a local FFL. Giving it a serial number on your own would run afoul of many new ‘ghost gun’ laws that have been put in place.

This may be true for specific US states, but declaring it wholesale true in the US is wrong. From the ATF FAQ on homemade firearms:

You do not have to add a serial number or register the [privately manufactured firearm] if you are not engaged in the business of making firearms for livelihood or profit.

I didn’t declare it wholesale true in the US.

If you weren’t jumping on your replies like some addicted meth head, you’d notice that I added some additional clarification about 20 seconds after I initially posted.