According to the tool I linked above, there are about 200 CSU to UC transfers admitted last year. A much smaller number for the OPs target schools. The total transfers admitted were more than 50,000. I don’t think it is a stretch to call 0.4% of total admits rare.
Don’t interpret that as discouraging you from applying - just understand it is not a well worn path. You have to align with your target school early and make a compelling case for your admission.