Electricity sources are tracked by the California Energy Commission, the 2024 report [0] shows the zero-carbon sources that make up this claim are:

  Nuclear     9.92%
  Large Hydro 11.06%
  Biomass     1.94%
  Geothermal  4.60%
  Small Hydro 1.52%
  Solar       21.30%
  Wind        11.89%

2% from biomass is suspect as a "zero-carbon" source, but the rest seems to be reasonably be zero carbon by today's standards.

[0] https://www.energy.ca.gov/data-reports/energy-almanac/califo...