UNLOADED open carry is legal, holstered or not. NO AMMUNITION CAN BE IN A POSITION TO BE FIRED. No filled magazines in well, none in cylinders, none in chamber.....none. At no time can the handgun be concealed in an manner or level. The seatbelt does not conceal a holstered handgun.
When within 1000' of a K-12 school that is currently being utilized for teaching state courses, ALL CONCEALABLE FIREARMS MUST BE LOCKED IN A CONTAINER. There is no definition of container, but a keyed or combo lock should seal the container (locking the trigger of the firearm is not considered "contained", and has no legal standing for transport. Trigger locks are for exiting the FFL after purchase). The trunk is considered a container, when there is no access from inside the vehicle. There is no need to contain a handgun when locked into the trunk with no inside access.
Ammunition can be carried in any manner you feel is best or easiest for you. Piling it to the roof in the back seat.....is legal. Ammunition can also be in the same container as the firearm, and can be in filled magazines. However, absolutely none can be in a position to be fired.
There are no California longarm transport laws, other than being unloaded. (registered Assault Weapons must be transported in a locked container) Federal law says transport in a 1000' K-12 school zone, must be containerized. That law was originally found to be un-Constitutional, was re-written, but has not been court challenged yet.