The TET routes are generally a mix of routable roads and off-road tracks.
I.e. Linking off-road tracks together via routable roads.
I have no experience with OSMand so can’t comment on that.
The TET tracks are freely available so in theory you could import them into
OSMand and see what you get.
The Garmin XT can handle off-road tracks but the Here maps can’t as far as I know.