There's a lot of discussion in the stickied thread at the top. The Ca DoJ/Attorney General site does a pretty good job of covering things, there's a summary here. It's not entirely current.
https://oag.ca.gov/sites/all/files/agweb/pdfs/firearms/pdf/cfl2016.pdf
A quick search will turn up the Handgun Safety roster. A handgun must be on the roster to be available for transfer via FFL (like sales, imports, etc.,) in Ca. However there are some exceptions. Non-roster firearms can be transferred by private party transfers. There are some people, mostly LEOs, who are not restricted by the roster. Once they legally possess the handgun, it can be transferred. Several have gone to jail for being in the firearms business without a license by selling too many off roster handguns. A person moving to California can bring their firearms. There is a time frame within which they are supposed to report them. They can import off roster handguns and subsequently transfer them. There are intra-familial transfers possible. "The infrequent transfer of firearms between immediate family members is exempt from the law requiring PPTs to be conducted through a licensed firearms dealer . For purposes of this exemption, “immediate family member” means parent and child, and grandparent and grandchild but does not include brothers or sisters."
Note there are legal requirements for PPTs. Also federal requirements would apply if the transfer is from an out of state relative.
One can not legally import assault weapons and a few other sorts of firearms that might typically be legal elsewhere (definitions available at the stickies above, etc.) nor ammunition feeding devices over 10 rounds. So, an off roster handgun must also not be an "assault weapon" under Ca definitions. CCWs are covered under state law but administered at the county level and maybe a few local chiefs of Police (not remotely common). Some counties, an impossibility, others fairly straight forward.