The folder is "Routes".
Just to digress for a moment, this will contain the routes that have been manually (ie cable) transferred as I understand that the routes done the normal way, ie through the app) actually reside in the app/phone.

I just tried a gpx file, as this is what I normally use and OSMand only gives the option to save as a gpx file.
Wahoo does support other file types, such as TCX (gives turn by turn) but these files need to be created or edited in specific apps/websites such as ridewithgps. In that case, you're online already, so no need for the cable.
I've had no need for turn by turn directions, but I have tested the idea of inserting my own cues for example "Campsite ahead, next one 20 km" so I can think about if I want to stop or go on. But I'd only be doing that level of planning at home, not on the road.