When you're on windows and not in the browser, you double-click to launch a file or program in the Explorer (which also is what runs the desktop). Single-click is select.
So, the rule:
List of files on your computer or desktop? Double-click. Otherwise? Don't.
What if I’m opening an email in Outlook? What if I’m looking at something in Control Panel? (That one’s a trick question, since the answer has changed in modern Windows versions.)
> When you're on windows and not in the browser
So many people have absolutely no concept of different windows let alone a browser. They run Chrome or IE maximized and that is "the Internet". They'll have tons of tabs open because they don't understand tabs and how to navigate them or that they can be closed.
A problem with billions of people using computers is that only a tiny fraction have working knowledge of them, an even smaller fraction understand them. Most people only understand operations by rote.