You can purchase a serial stamped, legal, traceable lower reciever/frame, and then purchase all the rest of the components of a gun, and assemble the whole gun yourself.

This is fairly common amongst experienced gun enthusiasts who prefer specific brands or designs for various parts, and like to do their own custom builds.

The result is a totally legal, non ghost, homemade gun.

...

Long Explanation of all the metal FGC9 parts an average person cannot make at home, period, or metal parts you can make at home but would need to have a CNC machine and significant machine shop experience.

The FGC 9 that you linked an article about... yes, it does feature more 3d printed parts which are typically made of metal... but it still requires you to buy many various metal parts.

https://www.hickoryhillarms.com/post/building-the-fgc-9

So even with this thing, here's all the parts that are not 3d printed plastic, that you would be very difficult even for an engineering graduate to create on their own unless they had access to their own industial machining tool manufactory:

Fire Control / Trigger Mechanism; Springs Disconnector Pin

Hammer Hammer Spring

Grip Screw Grip Screw Lock Washer

Feed Ramp Screw

Mag Catch Spring

Primary Buffer Spring Secondary Buffer Spring

Brace Screw

Ejector Screw

Alan Key / Wrench

Firing Pin Retaining Screw Firing Pin Retaining Screw Nut

Reciever Screw

Firing Pin Firing Pin Screw

... Phew. Ok, so, sure these parts are not that difficult to purchase, why bother listing them all?

Because you said you don't need to be an engineering graduate to make the metal parts of a gun.

That's not true for all the above parts.

You'd need to have an entire manufactory to make these things out of the material required, at the quality required.

...

The following parts actually could be CNC'd by someone with moderate experience with a CNC machine, and a CNC machine at home, but they're not made of 3d printed plastic:

Bolt

Barrel (Non Threaded, thus significantly innacurate at range)

Now, if you are even more experienced with machining, you may be able to produce a threaded barrel...

... But at that point we are talking about an experienced machinist with pretty uncommon equipment, which itself can be traced.

Either way, you can't make the bolt or barrel out of plastic for the FGC 9, and while yes, a novice machinist could learn how to machine one at home, the vast majority of people who build FGC 9s purchase the bolt and barrel from someone who runs a small, often psuedo legal business of making them.