"Here's an image, guess what is supposed to happen when you click it, also i will give you no info on hover over -- you must push me and then try to guess what i did!" (btw if there's a bug or a configuration state in the app that you or i am unaware of -- you might've pushed me at the wrong time and i might've let you do that)
The Apple II was the first computer that was widespread enough for people to have seen that had a floppy disk. It came out in 1977. Apple dropped the floppy disk in 1998 (21 years later), so it's been 22 years since it was standard in Macs.
Even allowing for some switching time, it has meant "save" for longer than it was a common physical object.
But if i really have to come up with something, I'd say green arrow pointing up. Meaning 'upload to your (own private) cloud'
Windows 10 introduced the navigation bar, which is essentially a vertical version of the Windows Phone application bar, just using a "three lines" icon as the "show labels" button instead of an ellipsis. https://docs.microsoft.com/en-us/windows/uwp/design/controls... The start menu and inbox mail app use this control, for example.
This is one reason why UI patents are not a good idea. When different platforms are forced to adopt differing UI patterns because they can't do what their competitors are doing, you get the current mess.
Back on Android, (or Windows, or Linux), it makes sense, whereas she loses her way. Just to point out, a part of discoverability I guess is familiarity with the underlying principles the UI follows as well.
To add two more points to this. I think there's one thing platforms in general got very right: simple touch controls. My one year old is discovering that sliding fingers across a screen, touching and pinching (the three basics) does stuff. Hidden menus, press-and-hold, and so on is secondary. Second, icons are something which is wrong. You need to guess their initial meaning, and translate badly in conversations, like recently with my mom: "press the rectangle with four arrows pointing out of it" (fullscreen or whatever it was).