Who Can Serve a Demurrer to an Unlawful Detainer

My question involves landlord-tenant law in the State of: California

I'm getting conflicting answers about this. A lawyer said anyone can deliver a copy of a demurrer to the opposing lawyer's office, even people party to the claim. A county clerk says it's a legal document and thus requires a non-party to eliver it and fill out a proof of service.

Does anyone know the rule here?

Thanks.

Bonus question: Does the demurrer need to be served to the plaintiff's attorney by the five day time limit to respond to the Unlawful Detainer?

Thanks again!

Who Can Serve a Demurrer to an Unlawful Detainer