The Ca. ban does not permit a person to build, buy, or transfer firearms that would fit into "pre or post" federal configuration, a post94 as well as a pre94 configured AR15 both fit into the description of what Ca. considers to be an "assault weapon" because their definitions are more strict.
Under federal law the AR15 was able to be reconfigured into a post-94 compliant configuration to slip by and not be considered an "assault weapon", reason being the criteria were "2 features or more" beyond the high cap detachable magazine.
Ca saw how it failed and re-worded their law so that a single feature beyond the high capacity detachable magazine was enough. That was strict enough so that an FAL, HK clone, or any semiauto with a high capacity detachable magazine and a pistol grip would not pass muster.
Hell, the Springfield M1A barely makes it because Springfield opted to remove the flash suppressor and install a muzzle brake instead. Hence the M1A gets by because it has zero evil features beyond the ability to accept a high cap detachable mag. Same goes for Mini-14/Mini-30s.
The Ca. ban allowed people to register what they already owned but they cannot transfer them to any other Ca resident, cannot will them to a relative, and in general if you want to get rid of one you either relinquish it to a local police dept or you sell it out of state.