In Android Studio I have two "flavors" of the same app, one is the free version with some functionality removed, the other is the full version with some extra bells and whistles thrown in.
When I am in the Google Play console, do I have to maintain two separate projects but just use the free-release apk for the free project, and the full-release apk for the paid version?
If so, how do I know ahead of time how to give a link from the free version to the paid version if I don't yet have any sort of URL or method for going there?
If you are referring to knowing before which URL should you add to your free app to redirect users to your paid app, then you can use the general convention that the Google Play employs
https://play.google.com/store/apps/details?id="packagename"
So you can give your packagename of your full app