Aerated concrete / brick / monolithic construction - who is familiar with it?

So I shouldn’t worry about the material of the walls [...] with the motto: in the end it doesn’t matter and everything is somehow the same?

Not everything is the same, but: the less special your requirements are, the less relevant the mostly rather marginal differences are. Therefore in the end: yes, (here) don’t waste any thoughts, you still have plenty of opportunity to do so in dozens of other places.

but just take what he usually uses?

Yes, because besides sound and thermal insulation, every brick has a third "dimension": namely the routine of its installer. Bake a cake exactly as it usually succeeds, just with different dough. That’s how it is with masonry too

: I am pleased with the successful example that one can write about construction material differences without hysterical fuss.

It is said that this means you don't need a ventilation system

The choice of the stone has no influence on the airtightness of the building. Regardless of the type of exterior walls, ventilation is necessary, either manually or through window rebate ventilators (usually uncontrolled) or through controlled residential ventilation (Controlled Residential Ventilation), the latter is available centrally or decentrally.

So I shouldn't worry about the wall material but just take what he uses? Sort of like: in the end it doesn't matter and everything is somehow the same?

I agree with my predecessors.

With the developer, it is usually ideal to stay as close to the standard (of the developer) as possible. The less you deviate from it, the better (in terms of the total price, but also in terms of the smooth running of the whole project).

Here you should carefully read the construction and service description (Construction Service Description), note the most important features (energy standard, heating, windows, sanitary, electrical, floor coverings, painting work, outdoor facilities) in bullet points and then compare (google other construction service descriptions). And of course determine what is missing compared to your own requirements.

I would also strongly recommend having the relevant construction service description checked by a professional!

Then (if everything is okay by then), you should also have the purchase contract professionally reviewed. Only after that should the signature follow.

The choice of the stone has no influence on the airtightness of the building.

You misunderstood that. The discussion was about KfW55, and there may be packages that achieve it monolithically only with a concept including controlled residential ventilation. Not every provider has all options on the menu; some turn the heating concept screw for KfW55 instead of the wall thickness screw.

With the developer, it is usually ideal to stay as close as possible to the standard (of the developer). The less you deviate from it, the better

This also applies to general contractors / construction managers. So it is not specific to developers.

Can you tell me what kind of specialists these typically are? We are planning to do the contractual and also content-related review anyway, I just don’t know exactly where and how to find the right specialists for that (and what costs to expect).

Experts, that would be an architect, a civil engineer, a master bricklayer, don’t you know anyone who could do that for you for a small fee? Karsten