Confusingly, "iCloud" and "iCloud Drive" refer to two different things.
"iCloud" is a mechanism by which your app data is synced across all your Apple devices. If you have it enabled in GoodNotes on all your devices that are logged in with the same Apple ID, you should see the same GoodNotes data on all those devices (that is, your data is synced across all your devices). It's not possible to locate iCloud data; we only know it's safely stored on Apple's servers.
By contrast, "iCloud Drive" is Apple's cloud storage (like Dropbox, Google Drive, OneDrive) that is part of iCloud and counts toward your iCloud storage. To see your notes in iCloud Drive, you will need to manually export them to your iCloud Drive by choosing "Save to Files" after tapping Export in GoodNotes.