CA law requires registration of "ghost guns"

[Grant H.]

Doesn't putting a serial number on it get into that whole "producing firearms" without an FFL thing?

Only if you're selling them. Just making them and serializing them doesn't mean anything on its own.

It's actually even more of a gray area than that...

It's only a problem if you manufacture them with the INTENT to sell them. It is actually completely legal to manufacture a firearm for personal use, serialized or not, and then later decide to sell the firearm.