Warranty and movable goods

Wasn't it the case that VOB can be effectively agreed upon if it is handed over (completely)?

Yes, it even has to be, otherwise VOB is not considered to be effectively agreed upon.

Regards from the Rhineland

Precisely because VOB is not "balanced" for consumers, the Building Code applies in favor of consumers.

Yes, it even must, otherwise the VOB is considered not effectively agreed upon.

To avoid misunderstandings. Yes, it is possible to effectively agree on VOB with a consumer and then the rules of VOB also apply. Is this statement correct?

Not so absolute.

If a consumer is a contracting party, the content control of general terms and conditions according to the Building Code applies as a corrective and takes precedence. It may "invalidate" clauses that are disadvantageous for consumers.

*Edit:
Simply handing over is not enough for consumers inexperienced in construction.
The contractor must then also comprehensively inform the client about the disadvantages and be able to prove/verify that this explanation took place. In particular, the client must agree to the inclusion of the general terms and conditions (here: VOB/B) (§ 305 II Building Code).

The VOB are furthermore not legal norms/laws, but general terms and conditions, which must be agreed upon as such. The VOB targets persons experienced in construction. Merely handing over the VOB documents does not convey this experience in my opinion. And as mere general terms and conditions, they are subject to content control according to § 307 Building Code.

For further study, copy and search, presented there well and comprehensively:

7th Symposium on Construction Law of the Chamber of Crafts Dresden vob

There, the presentation also addresses at the end the reduction from 5 to 2 years and maintenance.
Since in the case of the TO here the VOB was modified and not agreed upon 1:1, the Building Code applies in my opinion in his favor again, so unanimously 5 years for the building structure.

A big compliment for the detailed legal answers.
It even says for me that a VOB/B copy was handed over, but I did not receive anything at all, no instruction or clarification ever took place.

As I understand it, even without a maintenance contract, the 5-year warranty applies, BUT NOT for parts of mechanical and electrotechnical or electronic systems where maintenance affects safety and functionality.
Do I have to conclude the maintenance contract exclusively through the GU?
Is the GU obliged to offer me such a contract so that he can even comply with the 5-year warranty?

As I understand it, the 5-year warranty applies even without a maintenance contract, BUT not for parts of mechanical and electrotechnical or electronic systems where maintenance affects safety and functionality.

Now you got it ...!

Do I have to conclude the maintenance contract exclusively through the main contractor?

No.

Is the main contractor obliged to offer me such a contract so that he can comply with the 5-year warranty at all?

No. You conclude it – if you wish (but in my opinion it makes sense) – with the executing company (subcontractor of the provider). Rheinische greetings