Obviously they don't use toner and ferric chloride. I know the solder mask provides some extra niceties, but the traces are so tiny, and there are NEVER any mistakes. How is this possible? Do they... route? Mill? Are those real words? Bear with me, I'm new to this (I'm gonna use that excuse forever!)
Usually a photographic process for the copper, and silk screen for the solder mask and component legend. The copper can be done with silk screen for designs with wide traces and clearance - usually single sided.