Legal Question in Civil Litigation in California

If you don't know what you're doing, you have no business filing lawsuits in pro per. All papers filed with the court must be served on all other parties, or their attorneys, by someone over age 18 who is not a party to the case. Likewise, you may assume that the defendants' attorneys will serve the other defendants with any papers they file with the court. Consult an attorney.

Each party must serve all other parties with copies of anything he/she/it files with the court. That means each defendant must serve its papers on the other defendants as well as the plaintiff. But a party that hasn't appeared (filed any papers or shown up in court) yet need not be served. When it eventually does appear, it may request copies of prior pleadings, motions etc. from the other parties.

The plaintiff is responsible for providing each defendant who appears with each other defendant's first appearance papers, such as demurrers or answers. After that each defendant serves their own papers on the plaintiff and each other. As for the first part of your question: a pro per plaintiff is not required to speak only to the defendant(s) attorney(s). First off, the no-contact with represented parties rule only applies to lawyers, and second, it doesn't even apply to attorneys who are representing themselves.