After numerous unsuccessful attempts using ⌘F (Finder) and Spotlight, I switched to
Find Any File and searched for content containing DuckDuckGo (my chosen search engine) and a file name ending in .plist which turned up several files. I then opened the suspects in Xcode to confirm it was the file I was looking for.
That technique should work in almost any version of MacOS. If Xcode isn't available to verify it is the correct file TextEdit should work.