All defendants' demurrers to all causes of action asserted by the plaintiff are sustained without leave to amend?

Not always. Sometimes the judge will allow the plaintiff x days to amend the complaint.

a demur - is an objection that an opponent's point is irrelevant or invalid.

Lawyers informally define a demurrer as a defendant saying, "So what?" to the pleading.

If a cause of action in a complaint does not state a cognizable claim (for example, the claim is nonsense) or if it does not state all the required elements, then the challenged cause of action or possibly the entire complaint can be essentially thrown out (informally speaking) at the demurrer stage as not legally sufficient. A demurrer is typically filed near the beginning of a case, in response to the plaintiff filing a complaint or the defendant answering the complaint.

all causes of action asserted by the plaintiff are sustained without leave to amend - the case is dismiss and the plaintiff can't correct it.

essentially thrown out

Is this what you wanted?

OK. Court is adjourned!