1. Are they legal?
2. Rifles?  Pistols?  Loaded?  Unloaded?
3. Fully enclosed trunk?  Hatchback?
4. Resident?  Nonresident?

School me.

https://oag.ca.gov/firearms/travel

Right from the horse's ass's mouth/website.

https://www.friedmanhandguntraining.com/CCW_map_CA.html   FOR CALIFORNIA RESIDENCE

Transporting Firearms in California
Home Firearms Transporting Firearms in California
HANDGUNS
Pursuant to California Penal Code section 25610, a United States citizen over 18 years of age who is not prohibited from firearm possession, and who resides or is temporarily in California, may transport by motor vehicle any handgun provided it is unloaded and locked in the vehicle’s trunk or in a locked container. Furthermore, the handgun must be carried directly to or from any motor vehicle for any lawful purpose and, while being carried must be contained within a locked container.

Pursuant to California Penal Code section 16850, the term "locked container" means a secure container that is fully enclosed and locked by a padlock, key lock, combination lock, or similar locking device. This includes the trunk of a motor vehicle, but does not include the utility or glove compartment.

SHOTGUNS AND RIFLES
Nonconcealable firearms (shotguns and rifles) are not generally covered within the provisions of California Penal Code section 25400 and therefore are not required to be transported in a locked container. However, as with any firearm, nonconcealable firearms must be unloaded while they are being transported.

REGISTERED ASSAULT WEAPONS
California Penal Code section 30945, subdivision (g) provides that registered assault weapons may be transported only between specified locations and must be unloaded and stored in a locked container when transported.

Pursuant to California Penal Code section 16850, the term "locked container" means a secure container that is fully enclosed and locked by a padlock, key lock, combination lock, or similar locking device. This includes the trunk of a motor vehicle, but does not include the utility or glove compartment.

1. Generally speaking, yes.

2. Any firearm you can legally possess.  Must be unloaded.

3. Trunk or some other external locking compartment.  Storage space in hatchbacks, station wagons, vans, etc. don't count.  In that case, or anywhere else accessible to occupants, must be in a locked container if a handgun.  Must be in a container, period, for all guns.  Must still be unloaded.  Gloveboxes and similar locking interior compartments don't count as containers for this purpose.

4. Doesn't really matter, as long as what you have is legal for you to possess in CA.  If your firearms are taken by LE for any reason, as a non-resident don't expect to be able to get them back.  You might have a shot if you have several grand or more to sue for your stuff back.  Maybe.  Also, non-resident means nothing is registered, so if you illegally CCW, it's a felony right off the bat, while for those with registered firearms, on the first offense it's a misdemeanor.

Mags and such can be loaded, just not in the gun.  They don't need to be locked, be made inaccessible, be separated, or anything like that.  They can even be on your person.  10-round mags or less only.

Make sure to bring your own ammo.  You won't be able to buy any here.

CA laws partially strip you of your 4th Amendment rights when it comes to firearms.  They can search for firearms if they believe you have them to determine whether or not they are loaded and otherwise being correctly transported.  Do not tell LE you have firearms, give them clues that you have firearms, or otherwise inform them.  You have no duty to inform, and lying is not typically a crime.  At best, nothing good can come from LE knowing you have a firearm.  Do your best to avoid that.

My California truck gun is a Ruger American Ranch in 5.56 with a 1-4X optic and 10 round P-Mags.

Now how does this apply to AR pistols?

What about 80% AR pistols?

Resident or visitor?

Resident.

Same rules as for other handguns, although it should be noted that AR pistols need to be fixed-mag since it is impossible to make them featureless.  Those that are RAWs are treated like all other RAWs.

My concern with a "visitor" would be the difficulty of establishing it's not an "assault weapon."  So I would suggest someone just passing through might be better off with more conventional appearing firearms.  Internet blather notwithstanding, if in a suitable locked container, unloaded, who is to know?  If one is the innocent, well-behaved sort that just somehow attracts attention?  mmmh.