County Laws Supercede City Laws because Counties are legal sub-divisions of a state, Cities are not considered sub-divisions of a State. Neither can conflict with CA; with the exception of Prop 64 re: land use. For example, I don't believe a california county or city can ban Medicinal or recreational mj use.
In regards to CA Proposition 64, the State of California is to respect a county or city's decision to ban cultivation of MJ, production of MJ, sale of MJ (as it pertains to local land use "laws")
Not a lawyer, but that's my understanding.