As @tthew says the device will appear in Explorer/Finder as a flash drive usually called “Garmin”. On the device is a folder again called “Garmin”, open this up and you are at the level where you can start to see things.
Rides that you record are stored in “Activities” (could be runs as well as rides) and you can just copy these on to your machine and then upload them in to Strava or whatever from file rather than “from device”.
To copy routes to follow on to the device then copy the file (you can only do one at once unless the firmware has been updated) to “New Files”. When you restart the device it will validate it then copy it to the “Courses” folder.