what type of states politically/culturally would "Northern California" & "Southern California" be?

[Agonized-Statism]

If they split tomorrow? Both blue states anchored around San Francisco and Los Angeles with their own laundry lists of obstacles for Republicans. Population losses would be hitting NorCal much harder proportionally and the current problems in the tech industry wouldn't bode well for its economy, and while SoCal would have and maintain a larger population and a more diversified economy, its per capita personal income and income disparity might look worse. SoCal would inherit the oil and gas industry and Los Angeles' car culture, while NorCal would be freer to pursue an environmentalist agenda. SoCal's five UC and twelve CSU campuses, while prestigious, would struggle being disconnected from the rest of the university system. The North would inherit the Sacramento government, and the more interesting political developments would be in SoCal.

[Agonized-Statism]

It would be interesting to see how the environmentalist movements in Los Angeles and San Diego would evolve in a hypothetical independent SoCal.

While California receives about 75% of its rain and snow in the watersheds north of Sacramento, Southern California's urban areas have a more robust and spread-out water supply and counterintuitively are generally better prepared for drought conditions. The Bay Area doesn't really have the ample underground aquifers and groundwater resources of the South, and they would actually end up importing more water. Water infrastructure also isn't as good in the North, the reason being that 80% of California's urban and agricultural water demand comes from the southern two-thirds of the state dating back to the late 19th century. A highly engineered water system was built there to transport water to the most arid places in the state- at the cost of changing the natural environment and wildlife habitats, of course. SoCal's environmentalists wouldn't have quite as much pull with their state's better preparedness for drought conditions and reliance on the status quo of heavier water use, not to say they wouldn't be a much more regulated state than Nevada or Arizona. Their focus would be on combating their state's more extreme heat and air pollution, the worst in the country by far. But those issues affect poor and marginalized populations more than those that would be terrorizing all Northern Californians more equally- wildfires, floods, and water shortages- so we would probably see a stronger environmentalist lobby and more far-reaching legislation in NorCal.

You'd also have to wonder how both domestic and foreign immigration to OTL California would differ in this scenario, and how they would affect NorCal and SoCal respectively.

More immigrants would continue to flow into SoCal due to its proximity to the border. More Mexican-origin immigrants end up in Southern California and more Asians in Northern California, and that wouldn't change either. While the whole state is in decline for internal migrants, I would expect SoCal to bleed less than NorCal with the difference in economic prospects.

I know, I know, my SoCal pride is showing despite being way close to Oregon now. Not that I don't dislike the entire state at this point.