California is at-will. And it still requires 60 days notice for a mass-layoff, thorough a separate act, because it's a special case that's covered by a separate law.
> “an employer may not order a mass layoff, relocation, or termination at a covered establishment unless, 60 days before the order takes effect, the employer gives written notice of the order”
> “an employer may not order a mass layoff, relocation, or termination at a covered establishment unless, 60 days before the order takes effect, the employer gives written notice of the order”
https://www.dir.ca.gov/dlse/Cal-WARNAct.html