So, for example, everyone knew how to view a PDF. It's just that the author disallowed the most discoverable method.
They all knew how to right click, in the most discoverable way (trackpad), it's just that the author insisted they use the magic mouse instead, or know about ctrl+click which no one ever uses. That the magic mouse is a usability disaster is a whole other issue, but that's unrelated to OS X.
I havent used OSX in a bit, but the default app thing should be something that you can do through settings (at least Windows 10 makes it very easily discoverable that way). Apple, however, has always done a terrible job with default apps (for example, requiring users to change their default Browser, Mail app, etc through Safari, Mail.app and so on).
The Do Not disturb thing is ironically an iOS UI pattern that Apple has completely unnecessarily brought to the mac.
So are the invisible scroll bars. Mac OS X's scroll bars used to be easily visible (and more functional, because it immediately gave you an idea of the size of the view as well as allowed you to click anywhere on the bar and jump there directly).
Fundamentally, OS X has well described, discoverable and recommended UI patterns that cover all these cases. iOS and iPad OS don't. It's unfortunate that Apple has decided the HIG isn't needed anymore, but that's more of an Apple thing, than an OS X capability thing.