Is there any reliable, repeatable way to find a folder on MacOS?
https://support.apple.com/kb/PH13433?locale=en_US explains better about file save location for the app. If you're doubtful, open Logic, try to Save a new project and remember the location.
Replying to your: "whatever unknowable scope it's searching in", there are two places where it is written that "This Mac" is being searched. So see there to find the scope.





You can add the folder Logic(if and when you find it) to the sidebar by dragging the folder to the Favourites. 
You can go to Finder Preferences and set the folder under General -> "New Finder window shows" as your folder Logic. But you'd have to open a new window for that, so remember that shortcut as per File Menu in Finder.