I downloaded the thorsten medium and wondered, what the difference to thorsten high would be?

Go into Settings -> Accessability -> Speech (at the very bottom; at least on Android 13) and then TTS :) hope that helps

How do I apply these voices, so apps use it?

I've driven the same general route a few times and even then, it has been quite reliable. As mentioned, it can depend on the cities and stuff, so you're best trying it out for yourself :)

I'd like to recommend 'Magic Earth' to everyone, who wants a privacy respecting Maps alternative with trafic data. I used it on several >500km trips and it only misguided me once. It uses OSM maps and can navigate offline.