How are the maps produced
If you wish to edit/add to the map, create an account with openstreetmap and go for your life. It isn’t too hard to use, maybe a little tricky to start with. You can either use the potlatch editor or download JOSM.
Once all the tracks have been put in and appropriately tagged, you can then add routes. This connects the tracks into routes that are commonly ridden.
How to extract the route as a gpx file?
(eg http://osmrm.openstreetmap.de/gpx.jsp?relation=2263541 where relation is the relation id of the route that you wish to export.) This will export the route as a GPX route file where in fact you want a GPX track file. You can convert this easily using the gpsies converter.