The County provides a number of services to all residents of the County, whether they live in cities or in the unincorporated UnCity.
The County cites ((http://www.countywideservices.saccounty.net/Pages/default.aspx).) these coutywide services as things like voter registration and elections, criminal justice, MediCal and general assistance (welfare) and tax collections. The County Airports (Metro, Mather, Executive and Franklin Field) are examples of investments the County makes to help drive the areawide economy. The majority of these services are things that the public is reluctant to fund adequately, as they tend to reflect the underbelly of society (jails, criminal courts, welfare payments), unpopular services (property assessments and tax collections), and occasional or less-visible services (elections, marriage licenses). Yet the state makes counties provide services like this and, under the law, counties have no choice but to comply. An example is when Governor Brown proposed in 2011 to comply with federal directives to reduce prison overcrowding (AKA save the state money) by shifting prisoners from state prisons to county jails. This did not sit well with people like our own Sheriff Jones, but, ultimately, it had to be done. Counties, being subservient to the state, have to do what the state says and swallow the costs. The Covid-19 pandemic provided another example: the Governor issued executive orders - leaving county supervisors, county sheriffs and county public health people to carry them out.
Now some of our elves, being pretty good with numbers, have figured out that the UnCity subsidizes the incorporated areas. They say every dollar UnCity taxpayers pay in for municipal services brings back only about 40 cents of value for those services, i.e. most of what Arden Arcade taxpayers pay the County for municipal services winds up underwriting countywide services. It's a shame that elvish rule of thumb is very hard to prove, given the lack of transparency in the County's budget.