This isn't my experience with Co-pilot's suggestions. I've literally been able to have Co-pilot suggest a complete unit test based on a novel structure I hand-coded myself and a few words describing the unit test. The constants are often wrong, but it saves minutes of fidgeting with the syntax for unit tests and assertions.

These are not quotations from other people's code but something about the deep structures of language and programming language semantics. However, I suspect if you knew enough of a snippet from other source you could coax Co-pilot to suggest code learned from that source, but it would likely be washed over by other code in the corpus where it coincided with meanings.