Can’t remember if it’s Strava or Viewfinder where the route stuff is limited in that you have to do the route yourself first to be able to save it as a route. Something daft like that.
With a Garmin, you can use various Garmin tools (inc Basecamp) to turn a GPX into a Garmin route. Don’t need Strava for it, unless you want to grab someone else’s Strava ride in which case you either need Strava Premium, or ask the person nicely to send you a GPX.
Turn by turn is tricky with MTB as you can’t often correlate a turn with the actual location where there may be multiple tracks heading off from the direction it’s pointing in, and it’s can’t tell you to turn left at that tree, but not that tree, etc 😉