Ghost 9mm guns do not exist, as far as I know. You can have a stolen 9mm or a 9mm with the numbers filed off. But I don’t think anybody is producing 9mm guns without serial numbers privately, which is what a ghost gun is.
As far as AR-15s are concerned, it is relatively easy to manufacture a lower receiver from a billet blank or an 80% kit, and produce a lower without a serial number. This can then be combined with various other non-serialized parts to construct a ghost gun.
I think it is illegal to even have one of these in California. In other areas, you can have one on your property, but are not permitted to take it off your property for any reason.
Not true.
California now requires you to apply for, and obtain, a serial number before you can manufacture a gun on your own. Probably unconstitutional.
The federal law is you have the right to make a gun for your own use.
You cannot make them for commercial sale.
The latest interpretation that I know of, is after a year of ownership, they may be sold like other personal possessions.
Some claim you have to put a serial number on them.
Probably a good idea, as lack of a serial number draws attention.
But a serial number on a privately made firearm, without government registration as in California, is pretty meaningless.
You can get 80% Glock receivers