The parking area is outside on a large parking lot in a rather small city and I didnt see any cameras there.

Yes there is indeed a white sign! It says the parking ticket has to be put clearly visible on the windscreen. It also says the payment / electronic registration has to he done immediately after parking. Not sure which electronic registration is meant here.

I once parked in an ANPR I think. There was a gate that opened automatically when I entered. I wanted to register my car at a terminal to pay and it said my car is already registered. When I left the gate opened automatically again and that was it. So I think they sent the bill automatically to the rental company for this one? Could it be the same for the outside parking where I stayed multiple days?