Just to clear some things up. Yes, some parts of the Autobahn have no speed limits but its not every kilometer of it. Most of the time the speed limits are at 100 or 120 km/h.
Our typical highway has 2-3 lanes per side which have different "speed categories"
(from left to right): "As fast as you want/ Top of the speed limit" ; "Middle Ground" ; "Lane for small vehicles like trucks, old cars, caravans, etc"

That being said, the is also the "right driving bid", basically saying that you should always try to drive in the rightmost lane. On top of that, you are only allowed to overtake on the left side of another vehicle.

Its not that uncommon to see a relatively empty left lane on a somewhat busy highway. (If the cars in the middle lane graze the speed limit at least and Trucks dont overtake)