This Garmin Support article shows the steps to take to get Garmin BaseCamp to recognize your devices: https://support.garmin.com/en-US/?faq=ufgaLgXyiX2eSZfAsQhdu7&searchQuery=not%20recognized%20catalina. However, it doesn't say what to do when Garmin BaseCamp doesn't appear in the list.
I'm running macOS 10.15.7 and BaseCamp 4.8.10. When I plug in my Garmin 780, Garmin Express launches and finds the device. Any updates required are installed with no problems. Launching BaseCamp with Express running or not doesn't matter. When BaseCamp comes up, no devices appear. When I go to BaseCamp preferences>transfer, only an old device that I no longer have appears in the picklist. When I happened across the above Garmin Support article, I thought that would solve the problem. BaseCamp never asked upon launch to access removable volumes, so I didn't dismiss any window accidentally, and when I go to system preferences>security & privacy>privacy>files and folders, BaseCamp doesn't appear in the list. What do I do now?