Which prefabricated house company offers the sampling BEFORE purchase?

Well, Standard is a tricky thing. What did say once, "The water coming out of my Ideal Standard shower is the same as from a super expensive brand."

You have to keep that in mind.

I don't necessarily think Ideal Standard is bad; to be honest, I would prefer it over some cheap fake faucet from China on eBay.

But it’s my own fault, I want to understand (almost) every detail and be able to decide or approve the decisions of others.

If you really want that, then you are choosing the wrong method of building. What you want is only possible with individual contracts. Because building with a general contractor means receiving information at the level of detail "Anthracite windows, triple glazing, company xyz or equivalent.” Nothing more. You don’t know which RAL color the profiles have nor what values and properties they possess. The same applies to the glazing and the type of installation. And this applies to all trades on the construction site. What you desire unfortunately does not fit with a general contractor. At least that’s how I read it.

When I see how much work and how many hours of research I already have to put into the "project," I don't want to imagine how much effort it would be with a solid house.

Prefabricated houses are about 10% more expensive than equivalent solid houses.

We built with a regional general contractor (however extensively) and were able to sample and approve each trade in advance, and price increases for e.g. sockets were contractually fixed. Our scope of work description also did not include "or equivalent"; it must be clearly defined what will be installed. If a general contractor refuses to do this, he would be immediately out for me. Detail depth and a general contractor are therefore not mutually exclusive; you might just have to search more intensively for a company.

Prefabricated houses are about 10% more expensive than equivalent solid houses.

Can you really say that so generally? First of all, it makes a significant difference whether you take a standard floor plan or have a self-designed house built by a prefabricated house contractor. Secondly, most prefabricated houses cost the same everywhere in Germany, whereas regional general contractors should have noticeable regional price differences. That is at least my assessment, but of course it could also be nonsense.

Can you really say that so generally? First of all, it makes a significant difference whether you take a standard floor plan or have a custom-drawn house built by a prefabricated house builder. Secondly, most prefabricated houses cost the same everywhere in Germany, while with local general contractors the regional price differences should be noticeable. At least that's my assessment, of course it could also be nonsense

It is as well. Just look at the prices at Massa Haus for the same house type in Munich and Rostock. There are tens of thousands of euros difference.