• in map layers, you can select different ones; also google map. But this are only graphics for better information and doesn't have effects to route planning

• but in "options", you can select OSM, TomTom or HERE. This are different routing engines (with map data). You can plan a route by engine you prefer, then may switch to another engine to compare, if the route line is still the same. That's an advantage if you woul like to share your route to others, which uses different navigation devices.

In your case - if you want to transfer your route to your Garmin device - you should use the HERE routing engine for planning (map layer doesn't matter). But please use enough waypoints, because after transmitting to your Garmin device, still the device is going to interpret your route. This can end up in a slightly different route in the Garmin device...