The standard way to add tracks to a Garmin bike GPS is through Garmin Connect, a website with a mobile app. This requires an online account and a phone with Bluetooth to work. A family member with a Garmin Edge Explore was confused by this, and I didn’t feel comfortable uploading their GPS data to Garmin.
A simpler solution exists. You don’t need an online account or a phone to upload routes or download activities from the device. It’s easier and fully local, even though you lose some features like phone notifications on the device.
Finding a Route
You need a route that you want to ride. Route descriptions come as GPX, TCX or FIT files (there are others, but these are the ones I tried). Routes can be downloaded from different websites (such as RouteYou or Komoot), or they can be created (via OpenRouteService, BRouter, or Fietsnet). The result is a GPX (or sometimes TCX) file with the description of the route.
Riding a Route
Connect the Garmin to your computer via USB. It appears as a USB drive. Copy the route file to the ‘Garmin/NewFiles’ folder and eject the device from the computer. When ejected, a ‘Loading courses and locations’ message is shown. From the main menu on the Garmin, choose ‘Courses → Saved Courses’ and open the uploaded route. Tap ‘Ride’ and enjoy the trip!
You can download your trips when you are done. Connect the Garmin again and open the ‘Garmin/Activities’ folder, where each of your trips is saved as a FIT file. Use an application like GPXSee to look at the trip.