We trying to get a handle on this at work. We mandate Automatic Updates being on on work phones but when people come with issues they've rarely been updated.But... automatic updates take ages to actually download (can be several weeks after the actual release), and - correct me if I'm wrong, but it will never automatically update to a full new number version (e.g. from 16.x to 17.x).
I tested it last night. Phone told me about 17.1.1, everything said that it would be updated overnight it plugged in and on Wifi. Checked in the morning and it hadn't installed. It *had* downloaded and prepared - clicked "install", entered the passcode and I was straight to the apple logo but I still had to intervene.
As for my watch it had said the same, but needed to do more before it installed.