I think at this point it easier to list what isn't illegal in California.

The upper reciever is the load bearing part on an Armalite, but it's also not the regulated part. The lower receiver is, and all you need for that is a box that holds the trigger components in vaguely the right place, a hole that lines up the magazine, a hole to stick the buffer tube into at the back, and some way to nail the upper receiver to it.

You could carve an AR lower out of wood if you were dedicated enough.

It won't be accurate or be a long baller w/o a lathe to make a rifled barrel. Not that you couldn't mke a barrel with hand tools, it just would be very difficult.

And like others have said, only the lower receiver is the controlled item. You goto Brownels and can buy 98% of an AR w/o a background check.

Gah how can I forget about the shovel AK????

No its not illegal:

https://www.atf.gov/firearms/privately-made-firearms

You can't make a machine gun or other items on the NFA list though.

A homemade AR15 is completely legal to be made at the federal level.

The rest of your comment is off base.