You gotta understand something about the modern firearms industry that I think gets at your #1. In the past, a “gun” or “firearm” was a complete, assembled package of pieces. It was riveted, screwed, or bolted together, and you’d buy it as a pre-made set. During the Prohibition era where things like moonshiners and Tommy Guns were popular, Congress passed the NFA, which dug into the parts of a firearm that might be significantly more dangerous than others. Those included things like a) selective fire, b) barrel length, c) sound reducers, and d) portability. Suddenly a shotgun was legal, but a shotgun with a barrel sawed off to be shorter than 18" was illegal. A weapon that fired one round per trigger pull was legal, but a weapon that fired multiple rounds per trigger pull was illegal. A barrel with a device that shrouded the explosion was legal, but a barrel that silenced the explosion was illegal. A barrel sold as a non-handgun could not be converted to a handgun, which would make it illegal. The NFA was the first time Congress really tried to grapple with the multi-faceted definitions of “firearm”, and many gun enthusiasts can point out the idiocy of some of the weird attempts to define parts and pieces that fit one category but somehow not another.

Fast forward to the post-Vietnam era, and you find that suddenly you could disassemble and reassemble firearms to make new configurations. That includes changing barrels, lower receivers, grips, foregrips, stocks, etc. Suddenly you could shoot a .410 shotgun shell out of an AR-15 “rifle” simply by expanding the chamber and barrel to accommodate the larger diameter. You can also shoot a shotgun shell out of large caliber handguns. You can also shoot a pistol round out of a “rifle”, and a rifle round out of a “pistol”. The definitions are maddeningly confusing. In its attempts to try to stay ahead of the industry, the federal government had to decide which pieces counted as a “firearm” and which ones were just added furniture. The “lower receiver” is actually a “firearm” by law, even though it needs a barrel and stock to fire effectively and accurately, and it must have a serial number. The upper receiver is not a firearm because it’s just a barrel and a grip, so it can’t fire a projectile. Upper receivers are significant parts of a firearm, but they aren’t technically a firearm in and of themselves. So until ghost guns you could buy parts like Legos, and piece them together to make whatever kind of gun you wanted.

It was only a matter of time before someone realized that you can finish steps 1-9 in making a lower receiver, but if you don’t take step #10 it doesn’t fit the legal definition and therefore doesn’t have the same requirements. With a simple drill press and basic instructions someone can convert a block of shaped aluminum into a fully workable firearm. This is only a few short steps removed from the fact that anyone can use a bit of machinery to mill a lower receiver out of stock aluminum from scratch. Firearms are not terribly complicated to make as long as you have machines that are capable of producing pieces that fit snugly together with low tolerance (wiggle). A “ghost gun” is simply a bit of metal with most of the complex configurations of a receiver finished, but that needs a few final tweaks at home before it’ll fit with other pieces of a firearm. It therefore is sold without a serial number and significantly blurs the boundary between firearm manufacturer and consumer. They’re sold exactly as you’d think, as unfinished lower receivers.

In response to your #2, that depends on how your government defines “weapon” and “firearm”. You’re likely only buying a chunk of metal in the shape of a lower receiver, and if you don’t know how to mill it yourself it’s 100% useless as a firearm. I have no doubt you could import one to your home country, but your law enforcement agency would have difficulty figuring out what law you’re violating by doing so, because a law has to have a very specific definition to be actionable and enforceable.