You need to right click a mesh in your content browser and selct convert to nanite, or something like that… Beside that you probably want the highest resolution mesh from the megascans library, when you access megascans from the UE5 Editor you will get the option to import “nanite” version of the asset you want. Thats probably the highest resolutuion mesh you can get plus you no longer need to convert it.
Thats all i figured out so far… i would also recomend to watch some tutorials on youtube, since we all are bascially exploring UE5…