I remember when google maps changed from having a from location box and a to location box to just letting you type "from address A to address B", which is pretty nifty. However this was never actually documented so I spent months pressing randomly to get the old UI up (it still seemed to exist, but only in some states of the application) before I finally realized that was the new way.
In iBooks you can download all the user guides for all devides
I don't know if there is or isn't official documentation everything you might typically want to know for Apple products. Clearly, they don't document the answer to every question that anyone might ever ask, but they do document many things that you might not think they do.
I know this because often when I Google for how to do something on a Mac, I often end up on some Apple page that tells me how to accomplish what I want. E.g., a page that documents a bunch of MacOS keyboard shortcuts, or what have you.
Of course, one problem is churn. I.e., the features in the software are changing frequently, and consequently, documentation can get quickly out of date. Also, Apple is likely to be cooperating with book authors. There is the "Missing Manual" series, which often seems to contain a lot of useful information (though, as I mentioned sometimes out-of-date), and I'm sure that David Pogue and friends don't figure all of this out via just trial and error. It seems like they must get some sort of assistance from Apple in the form of release notes or rough documentation, or something.
Apple products have been like this for ages. I.e., they present a "simple" interface for the "typical" user, and then they provide "advanced" features for the "power user" that are only accessible via keyboard shortcuts, or what have you. (E.g., doing a screenshot on MacOS is via Command-$.)
Their success in achieving the right balance, however, has been quite mixed IMHO.
How do you find out about the "advanced" features? Via Google or Stack Exchange or reading a book or guide, I suppose.