Brother wants to move to CA

My brother is interested in moving to CA and has a few off roster pistols he was thinking of getting rid of before the move but I was explaining the roster to him and he was thinking about just bring them over in case he wants to have a few cool off roster pistols while he’s living here for the few years or possibly get rid of them down the line to make his money back as he in his state the resale value is far under MSRP as they have no roster lol.

My questions is, how does he register the guns over here? Does he need a State ID/ does his current one work? He wants to do it ASAP so there’s no laws he’s breaking for bringing an off roster gun or unregistered to the state firearm? not sure the situation wants to see if anyone here has experienced this.