What’s the difference between Save, Save As, and Duplicate in macOS apps?

Even for long-time Mac users, it can be challenging to identify the differences between Save, Save As, and Duplicate. Save is seemingly straightforward, but not necessarily so—a lot happens behind the scenes for every save in Pages, Keynote, and Numbers, among other apps from Apple and those that adapt the Save/Duplicate/Save As approach.Save
For apps that support storing versions of files, each Save not only takes the current changes stored temporarily for an active document and writes t