“Old” apps rely on mouse pointing and tooltips to explain what a given button is for. Maybe there is text associated with the button, but you can only put so much text without wasting useful screen state. More so in a phone or tablet app.
I’ve seen a trend in Google apps where they put an overlay on top of the app the first time it runs, to explain how the various pieces of UI work. I have seen this also on my Nexus phone, but don’t have a screenshot. I don’t recall a way to actually bring that help overlay back again, so that’s maybe some built-in gesture that’s missing. Here’s what it looks like in Gmail, the first time you use the new compose layout:
I like the approach very much, and I think it’s something that should become standard part of mobile OS, and Windows, including a standard way to bring that help up from within any app.
Update 11/22: here’s another example, now from Netflix Android app:
Update 11/25: looks like a tend to me now. This time from YouTube app