Those keyboard shortcuts are fairly discoverable because they are actually menu item shortcuts: Application>Quit and File>Close. The idea with macOS is that the

Those keyboard shortcuts are fairly discoverable because they are actually menu item shortcuts: Application>Quit and File>Close. The idea with macOS is that the menu bar is easy to get to (being at the top of the screen) and has most of the commands you need. It's also fairly standardized across apps. Originally Mac apps didn't even have many keyboard shortcuts because Apple wanted to encourage app designers (and users) to make use of the mouse and menu bar, rather than just recreating something like Wordstar. I think user tests also showed that the menu bar at the top of the screen was fairly quick to use and not drastically worse than keyboard shortcuts. I will admit that placing "Close Window" under the File menu in Safari (for example) rather than the Window menu seems counterintuitive. The reason is that Mac apps are traditionally document-based, the File menu deals with document operations, such as closing the current document. I believe it also predates standardization of the Window menu. Unfortunately Safari (and some other apps) aren't exactly document-based, yet they share the document-style menu layout for UI consistency. Personally I wouldn't mind duplicating Close in both the File and Window menus.