I'm not a lawyer, this is not legal advice:
In CA, you are required to have a CA CCW to carry concealed. Illegal Concealed Carry is kinda spelled out here:
https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=PEN§ionNum=25400. 1st offense looks like a $1000 fine, or jail time.
To transport firearms in CA, they need to be unloaded and in a locked container (or have a cable lock through the chamber). The locked container cannot be the glove box, center console, or other utility box.
https://oag.ca.gov/firearms/travel#:~:text=Pursuant%20to%20California%20Penal% 20Code,or%20in%20a%20locked%20container.
To me, this looks like, if the gun is concealed on your passenger's person, you would most likely not be criminally liable, but would most likely give PC to have your vehicle searched if stopped, and if the LEO discovered the gun.
If it was in the car, and not on your passenger's person, I think you are more liable (responsible) for the transportation of it, as it's your car and you're driving it.
It also gets weird if you're camping, you're allowed to concealed carry in your dwelling (car/ motorhome / tent/ campsite). sorry can't find source for this.