Now, about the CHALLENGE of putting together such a gun kit, the proper term for which is 80% Firearm Kit. Ghost Gun is a politically charged term used to frighten non gun owners. So, in order to finish such a firearm kit one must be in possession of very specific and often expensive power or power bench tools and also possess some degree of mechanical aptitude. Regardless of any included directions the weapon to be completed must be machined to tight specifications if one plans to have a functioning firearm at the end.
At the end of day, however, a fellow American's firearm ownership is none of your business. Keep that in mind.
Just for clarification, the term ghost gun generally refers to any gun made for personal use that doesn't have a serial number. Many such guns are, indeed, made from 80% kits; many are not.
For the OP, an 80% kit is not at all an 80% gun. It is a gun receiver less than 80% complete. The 80% number is an ATF regulation that says the frame must not exceed 80% completion to be sold as, basically, a chunk of metal instead of as a gun. Those kits might be 10% complete, or any thing in between but cannot exceed 80% complete.
Even the 80% completion is not that simple. The incomplete portions can't be just any 20%; the undone portions must be significant to the operation of the gun and require significant skill and equipment to complete. The undone portions can not be something that can be resolved easily by just searching the Internet.
Many people build their own guns using forged frames or receivers, leaving 90+ per cent of the work yet to be done.
Many other people buy a chunk of aluminum or steel and, having the skill and equipment to do so, build their own from scratch.
But you're right, night_son; in the end it's just no one's business at all what guns someone has or builds. Adding a serial number to mass produced guns was not the law for the first 179 years of our nation's history and we didn't have 10% of the mass shootings we see today. Guns aren't the problem; schools are.