Indeed, Garmin uses the Here map.
You can create a route in the TomTom map without any problems, as long as you compare the route with the Here map via the "Toolkit" - Compare route with... This is because there are map differences between Here and TomTom
But as an XT user, I myself use the Here-Map with the "Michelin" map layer over it, as @Jack already indicates.
People who enjoy running, clambering and jogging through their environment
RE: Here or TomTom map?
RE: Total ride time different when exporting to Garmin
Use the Here-map instead of the OSM map (Green line is the Here-line)