The buyer and the seller have their own closing costs that they are responsible to pay. In any county, the buyer can ask the seller to pay their closing costs in the form of a credit. Most buyer closing costs are loan fees and title insurance. The seller's are the commission and depending on the city and county, the transfer taxes.
There are standards for each county. It's true that in San Mateo County it is standard for the buyer to pay closing costs such as title and escrow fees and loan points. Sellers typically pay the city (if there is one) and county transfer taxes and the hazard report.