Deficiencies in the new construction. Dispute over the amount of withholdings.

No acceptance of the individual trades was offered, so I could not check the lines that were installed during the construction. According to his own statement, the developer accepted all trades from his general contractor. He would have had the duty to point out the error to his general contractor, wouldn't he?

Nope. He has a contract with the main contractor and one with you. He accepted the trade from the main contractor. If he then has a problem with you (because you do not accept the result, because it is different from what was contractually agreed), then the problem lies with the developer.
Why are you even talking to the main contractor, do you have any contractual relationship with him?

Yes, of course he would have. That is why he is also obligated to remedy defects or reduce the price.
I just meant that as a client you don’t necessarily have to trust blindly and usually also check for yourself if everything is correct, unless you also hire a construction supervisor during the build. It’s a different matter if you were not allowed to enter the construction site; I have no idea how it works with developers.

We initially visited the construction site spontaneously without an appointment, mostly during and after working hours, and at some point were asked in writing not to enter the site without an appointment anymore...

Copper is more corrosion-resistant and has lower pressure losses due to pipe friction than steel pipes. However, steel pipes are more susceptible to corrosion. For example, they are not suitable for areas that are exposed to moisture.

Are we talking about heating or domestic water? In houses from the 70s and 80s, there are often problems due to copper corrosion in water pipes. Does the steel pipe have such a much shorter lifespan? If, for example, you have to replace pipes after 30 years instead of 50, that is a significant defect. I wouldn't settle for 500€ in that case.