So, if so, would the California counties that want to remain in the USA be allowed to do so?
And would all citizens of California automatically become citizens of Mexico? What about the ones who don't want to? Just the registered Democrat voters?
Sate secession means the entire state secedes. If parts want to organize and form new states they do so at their own risk. Further dividing California would be up to them to decide that.
States secession means state secession.