Haha, yes! I'm a software engineer and it took me a decade of smartphone usage before I realized that long-pressing various keys (e.g. vowels, symbols) shows a pop-up allowing you to enter related characters (e.g. á, æ, ¡, ₹, ±, ·, ½, ²)†. Blew my tiny little mind! If I were using a language that required the variants menu in its native keyboard layout, I might have discovered this a decade before by explicitly searching for how to enter certain characters.
† Typed these on mobile, of course. Android, in case you're wondering.
Maybe related to being a SWE myself, but usually the first thing I do with any piece of software is go to the settings. If you do that with the Google Keyboard you'll see "Preferences -> Long press for symbols".
If you enable that, each key on the keyboard now shows an additional symbol you can get to by long press, for example, 'a' now also allows you to type '@'. If you try and long press for 'a' you'll suddenly see that accented versions of 'a' are also available. And that's the story of how I "naturally" discovered that same feature.
If I remember correctly, early Android versions (or it could have been HTC specific) had a tutorial for this, as part of the new phone setup process. I guess they removed it when most buyers owned a smartphone before.
Around the time of the first iPhone, Apple also had a long video on their website (it must have been 30-60 minutes) meticulously going through and explaining every feature of the iPhone. No idea if something like that still exists.
† Typed these on mobile, of course. Android, in case you're wondering.