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The etrex can use gpx track files and gpx route files, most garmin devices and most "route" creation websites create gpx track files. Therefore when you put the gpx track file on an etrex it does not give turn by turn directions , Which in my opinion is a good thing , especially when off road.
Garmin edge devices in general do not differentiate between gpx track and gpx routes and convert them to FIT files, for use on the device.
If you load a gpx route file onto an etrex it will stay as a gpx route and will give turn by turn directions, depending on the map loaded on the device being "routable".But I usually just use the pointer screen like yourself.
A gpx route file giving directions
How? I've always just followed the line on the map, which is fine unless its an unfamiliar route that crosses itself a lot