Back in the day, on my old Garmin Edge 705, I used a USB cable to import routes to follow while riding, and exported ride recordings after I returned home. Plugging in the Garmin to my windows PC, it shows up as a USB drive.

Routes: a route from ridewithgps.com -- exported as a .GPX file.
I no longer remember, but I think i copied the gpx file into \Garmin\routes ? There's a set of folders on the garmin, it should be easy to pick the relevant one. (Now, on modern Garmins, the files go into a \Newfiles folder instead).

Ride recordings: I copied the file out of the Activities folder, saving on the PC, then ridewithgps or strava has methods for importing from a file.