Open the person's activity, so click on it if you're looking at it in the home feed.
There's a button with 3 dots on the left hand side of the screen.
Click create route
Save the route with whatever you want to name it
Click Export GPX
Win
Even easier than that, assuming you don't want to edit. Whilst viewing the ride you're interested in, click on the star icon. This should then save it to your routes. Then select from Strava routes on the Garmin. No paid account.
Open the person's activity, so click on it if you're looking at it in the home feed.
There's a button with 3 dots on the left hand side of the screen.
Click create route
Save the route with whatever you want to name it
Click Export GPX
Win
I don't have a paid account and it works fine