Is the Stack Apps site part of The Stack Overflow Trilogy, or is it just one site of the
Stack Overflow family sites Stack Exchange sites?
Sorry, but I'm a little bit confused.
The Stack Apps support site is, essentially, a Stack Exchange site. It runs on the Stack Exchange engine. It provides Q&A for a community for product support, feature requests, and bug reporting.
The topic just happens to be a product of the Stack Overflow developers. But there's nothing to tie it specifically or exclusively to the Trilogy.
I'd say it's an essential element of the StackOverflow Quintology.