The tax treatment of bonuses is the same as that of any other pay income. There’s no difference.
If no wage base limits have been met, the expected amount withheld from a paycheck made as “supplemental income” in California will be 40.98%. This includes all forms of withholding taxes.
Depending on the tax bracket from which your basic wage is derived, my experience ranges from 40% to 60%. But come tax filing season, I happen to get at least 20% back.