I'm not totally au fait with XML parlance to know what you mean by a property sheet widget. My understanding was that a property was part of a widget in an xml file. If you meant something that tells you about all the available widgets .. the links below should assist you there if used together.
There would only be XML coding for an addon if the creator used them. It's an individual choice so some people do and some people don't for a variety of reasons.
As to Blizzards use of the XML file you might want to take a look at these files:
https://www.townlong-yak.com/framexml/live
And this is a good site to use as a guide to what is out there.
https://wow.gamepedia.com/Wowpedia:I..._customization
Hopefully these will assist you in what you need.