It depends what you mean by "center".
The geographic center, as listed here, is in Madera County, close to Fresno.
What about the "center" of population? As of 2020, it's much closer to Bakersfield:
This surprised me. It turns out that half of California's 38 million people live in the LA area! Only 20% of the state lives in the Bay Area, and less than 5% live north of Sacramento.
If you're curious, check out the full analysis. I also made a map for every state. Here's that analysis.
The data is from OpenDataSoft and World Population Review.