Can you share what app/device you use that has all of those features like specifically picking out fun roads? I'm not aware of that feature on any major nav app.
Garmin has that functionality on their moto-specific GPS units. I've used it on my BMW-branded Garmin a few times. It isn't going to find the equivalent to Deals Gap in North Dakota, but it does the best it can with what it's got to work with, i. e., if there's something with some curves, it will reliably favor that over a straight road.
Sounds like he's probably using a TomTom Rider - there's an option specifically to pick a scenic/fun ride between A and B.
I’ve found google maps offline mode + the avoid highways feature can have good results. It also tends to have shitty results in highly interconnected areas, but it works alright for the country.