The docs for expanded data types are good, but for a working example you have to go trolling through the array data type source code, which is enlightening but a pretty heavy lift for me especially if I have to come back to it after some time.
So I decided to distill what I could glean from the source into a simple, self-contained, completely useless extension called pgexpanded:
I'd like to ask 1) does this seem like a useful project to other people? If there is interest in me taking it further I'll expand it with some more documentation and pgtap tests. and if you think "yes" to 1, then 2) would those who know a lot more on this subject than I do would like to review the code? I'll put on my finest asbestos.
