If the route is stored on Garmin click the ... menu to download the GPX file. You don't need a Garmin device or account, but if you do, you can Send to Device directly from this screen.
If the route is stored on Strava, click the v button next to Duplicate to export the GPX. Alternatively if your Strava account is linked to Garmin, Wahoo or other device, simply click Save and you can then sync to your device.
If the route is stored on RidewithGPS, click the More button to Export as File and select the format you need - GPX is usually fine for all devices. You'll then need to load the GPX file to your device yourself.
If the route is stored on Komoot, you will need an account before you can download the GPX file. When that is set up you can click the Download GPX option or Send to Device if you want to navigate using the Komoot app on your phone or a Garmin device.
In your Garmin Account, go to the Courses screen and click the blue Import button (not the similar icon in the top right corner!). Drop or locate the GPX file, pick the type of route and finish saving the course. Send the course to your device and then from the device sync using using the WIFI or phone connection.
On your mobile, locate the downloaded GPX file using your preferred Files app and open it with the devices companion app. Pick the type of route and finish saving the course and then Sync the device.
If you use Strava on your phone to navigate a route, you will need to upload the GPX file to your Strava account on the Desktop site, since the app doesn't support opening GPX files. Create a New Route and click the up arrow button and save as usual. You need a Premium account to be able to do this.