You have to have a Upper/lower receiver to complete the process. They are legally a firearm and have serial numbers. You would need skill and machinery to complete that receiver if you were building from scratch.
Only the lower has a serial number on an AR.
That is the firearm according to the ATF.
The 80% kit is a lower.
Not as easy to build as an AK, but not that difficult.
The machine work is relatively easy. Glock pistols are easier than AR-15 lowers, “or so I’ve heard.”
“Somebody also told me” there are no serial numbers because they are legally homemade firearms. They are made for personal use and not for sale.
Ghost guns is another one of those cool “assault Rifle” terms.