How soundproof are windows?

Regardless of the quality (which is more or less identical anyway), the windows should not be as noisy as you described. Who knows how the installation was carried out... Feel free to install cheap "Poland windows" yourself (e.g. Plastbud with Saint Gobain triple glazing) and with proper installation, you won’t hear normal noises from outside.

I was once at a construction site with a customer, where the window area in a bedroom was about 8 sqm in one piece, and it sounded - with all elements closed! - like it wasn't there at all, even though only the seals at the transitions between element/wall and element/ceiling were not yet tight.

Laypeople regularly estimate completely wrong how much sound actually comes through the windows and how much actually comes around.

You have a PM.

I recently read in another thread about everything you have to pay attention to when installing windows. A lot can really go wrong, and there was definitely shoddy work done at our place. It's no coincidence that the developer is in court with the company...

Window construction is actually not rocket science - but there is simply no area where careless mistakes can be made without consequences.

But couldn't it even be the roller shutter box? What many also forget is that bushes and trees absorb a lot of noise; once everything has grown, it will be quieter.

Could it not even be the roller shutter box?

Yes, sound pressure "basically" prefers to propagate rather than be reflected. Overflowing is ultimately just a flowing around - the sound "mainly" wants to go past the obstacle.