Biggest thing: Detachable magazine
If you have a detachable magazine, you may NOT
have a flash hider, pistol grip, thumbhole stock, foregrip, bayonet lug etc.
According to the courts, if your rifle requires a "tool" to remove the magazine, it isn't "detachable", hence the bullet buttons and prince 50s and what have yous. IMHO, getting a bullet button is the way to go. Slows down mag exchanges, yeah, but at least you can have a halfway normal rifle configuration.
Also, some brands are verbotten. "off list lower" means a lower receiver not stamped with the name Colt, DPMS, Bushmaster, Armalite etc.
A CA FFL won't sell these to you, so that makes it a little easier to comply
Also, no mags capable of accepting more than 10rds, no barrels under 16" (NFA items illegal here, regardless of tax stamps), and so forth...
I am not a lawyer
, I'm not even handsome enough to play one on TV. Consult the CA DOJ website if you have any doubts!