What is the most cost-effective type of shell construction?

Any "intermediary" must be paid for their service, otherwise the way to the client is shorter than you and other builders would like.

...so is it always most expensive with house construction companies/prefabricated house companies or "everything from a single source"?

...so using a house building company/prefabricated house companies or "everything from a single source" is always the most expensive?

That is, as always, a question of what I expect. For example, I drive a car from the Swabian region, another prefers a Romanian version of a motor vehicle. In a crash, I might have dislocated my shoulder, and another, sitting next to Petrus playing the harp, thinks: I should have invested a few euros more.
The building expert and I know exactly that not everyone who wants to build can afford our (consulting) quality; that is why we voluntarily advise questioners here in our spare time again and again with the same questions such as:
Who has experience with...?
How can I save as cheaply as possible... do own work... cheap, etc.
We would also rather answer questions like:
How can I ensure getting the best quality for my money when building a house? This question so far has simply not been asked by anyone.

How can I ensure the best quality for my money when building a house? This question has not been asked by anyone so far

It is about the same house, which, as I interpret from your answer, is cheaper through an architect and individual offers from craftsmen? I don't understand the consultation part. Everyone has a certain idea of how the house should look. Now you want a price for that. A price with good/bad insulation, gas heating/geothermal, etc. Then you have a few prices and weigh what the "luxury" is worth to you or what you can afford. Then you look for the cheapest providers to carry out the exact planned project. Am I making a mistake somewhere? What characterizes your consultation? How do you differ from an architect's consultation? Did you also study?

In short form:

Does an architect give you a fixed price? Where does the architect buy the walls, from the local carpenter? If the construction costs exceed the architect's estimate by 30%, what do you do then?

The misconception lies in this: I calculate what the house cost when you moved in, you want to know what the house costs when you sign the contract. These two prices differ significantly from each other, and you will notice this after signing.

First, I check wishes and budget, which 99% of the time do not match. Then the client must make compromises; there are only three possibilities: house, land, budget. If he is not resistant to advice and there is a chance to build the house, we make an offer based on the scope of work. Then the client selects finishes for his house with all local craftsmen. Afterwards, our offer in each individual item is backed up by the corresponding craftsman’s offer, overall about 50 pages. We pass the craftsmen’s prices on to the end customer without any markup; we only take the 2% discount for ourselves if the client also pays his invoices quickly. If, contrary to expectations, the prospective builder is satisfied up to this point, we hand over our contract documents with the request to have them reviewed by a lawyer of his choice. If then the unlikely possibility exists that he still wants to build with us, we sign the contract and get started.

This working method obviously corresponds with that of other prefabricated house providers, doesn’t it?

In brief:

Does an architect give you a fixed price?
Where does the architect buy the walls, from the local carpenter?
If the construction costs exceed the architect’s estimate by 30%, what do you do then?

The misconception is this: I calculate what the house cost when you have moved in; you want to know what the house costs when you sign the contract. These two prices differ significantly from each other, and you notice that after signing.

I first check wishes and budget, which fits together in 99% of cases. Then the builder must make compromises; there are only three possibilities: house, land, budget. If he is not resistant to advice and there is a chance to build the house, we make an offer according to the construction specifications. Then the builder chooses materials and options with all local craftsmen for his house. Afterwards, our offer is underpinned in each individual item by the corresponding craftsman’s quote, a total of about 50 pages. We pass the craftsmen’s prices directly to the end customer without markup; we only take a 2% discount if the builder also pays his invoices promptly. If, unexpectedly, the prospective builder is satisfied up to this point, we hand over our contract documents with the request to have them checked by a lawyer of his choice. If then the unlikely possibility exists that he still wants to build with us, we sign the contract and start.

This method naturally corresponds with that of other prefabricated house suppliers, right?

The architect gives me the same "fixed price" as a house building company.
Where do you buy your walls? Sometimes I really think I’ve lost my mind? Where the architect buys the walls? Sorry, I don’t understand that. The walls are built, the mason makes an offer.
Why should construction costs be repeatedly exceeded, especially by 30 percent? The experts know what they are talking about, especially an architect. Please explain, I can’t understand. Doesn’t he know what the bricks cost that are stacked there? What about sanitary, electrical, floor coverings, etc.?
I’m no expert, but I can’t understand what is so complicated here. You have a floor plan, you know the exact dimensions, you know how many doors and windows, what kind of roof, what kind of heating, what kind of insulation and also what quality, electrical, sanitary, 2 toilets, a bathtub, tiles, floor coverings you can choose exactly, what kind of screed, etc. is underneath; you know it precisely, you can calculate everything exactly. So where is the catch that should then cost a third more?
So you pass the craftsmen’s prices 1:1 to the builder and live off 2 percent and apply no markup?

@Interessent:

How many houses have you already built?