Not much. All counties in California have a Sheriff (I believe they are all elected). Cities, OTOH, can have police departments that have sole jurisdiction/first jurisdiction within city limits. Except for San Francisco County, all counties have land that's outside of city limits. San Bernardino County has the largest amount of county land.
Sheriffs enforce the law and investigate on all county lands. In some places, sheriffs also run local police for incorporated cities (by contract). In most places, cities do not have jails, only counties do (and in some cases, it's in the next county). So Sheriffs typically run jails, perform medical examinations, take crime scene photos (even within city limits), run larger crime labs, and...have better training. I taught at a Sheriff's academy for several years. The amount of training needed to be an entry level Sheriff's deputy was quite high, but where I grew up, the city police didn't require any training at all. In fact, it was and still is the case that it's the people you know who get you a job in my California home town.
Unincorporated land, state land, federal land, unincorporated settlements - the 911 calls typically go to the Sheriff. Some federal lands (big National Parks) have their own LE, but still work very closely with the Sheriff from the surrounding area.
So, the City of Los Angeles (as an example) as its own police force, so does the City of Beverly Hills (and so on). But all the land that's not part of a city...LE is the LA County Sheriff.
Not sure how San Bernardino works, but I think the City of San Bernardino may contract with the Sheriff.
So you could be in a police station in, say, Simi Valley, and it would be run by the Sheriff as a substation, or you could be in a police station in Los Angeles and it would be run by City police.
