Corners of the new windows are very dirty or scratched

I see it very differently. Nothing is normal here. It could also be that the corners were damaged during assembly. It is also not customary for someone to come back and re-sand there. For example, I have windows laminated on both sides, that wouldn't work at all.

From the gut it also doesn’t look normal to me! I have Unilux windows at home myself and can take a look at the corners later. At least it wouldn’t have caught my attention that they look even remotely like that.

e.g. double-sided laminated windows, that wouldn’t work at all.

So? So the film comes directly from the extruder?

I can only say this about your statements:

You are on the wrong track.

Of course there are processes that make post-processing practically unnecessary. But that makes the window more expensive. Apparently here we are in the low-price segment. And there post-processing is necessary.

So? The film comes directly from the extruder?

I don’t know how you come to that conclusion. Judging by the pictures, the corners could also have been damaged during the installation of the windows, or during transport, or during glazing. In any case, after any possible film is applied.

Here we are apparently in the low-price segment.

It mainly looks to me like the "seller sends some random guy for installation" segment. Whatever was delivered gets installed - without checking whether it was manufactured flawlessly.

There shouldn't be any foil, in my opinion those are Unilux windows and here is the white painted wooden interior side. I would also classify it as mid-range and not low-budget. Sure, there are higher quality windows, but I don't think Unilux has anything to do with low-budget. The installation might be more like that! As I said, I have the same kind of windows at home and I would actually be surprised if the corners looked like that!