Projects advertised as prevailing wage covered projects usually have funding triggers or owner requirements that require the payment of prevailing wage. Prevailing Wages are set by type of work performed, or classification, and sets the minimum base wage (what is paid to the employee) as well as the overall hourly rate which is the base rate plus fringe benefit amount.
Projects can have State prevailing wage requirements set by the California Department of Industrial Relations, Federal prevailing wage set by the Department of Labor, or for projects that are considered dual-funded and have both requirements, the employees must be paid the higher of State or Federal wage rates.

Links to additional information:
CA DIR: Prevailing Wage Requirements (https://www.dir.ca.gov/public-works/prevailing-wage.html)
Department of Labor: Davis-Bacon Wage Determinations (https://www.dol.gov/agencies/whd/government-contracts/prevailing-wage-resource-book/db-wage-determinations)