A general contractor offers you a prefabricated house in the sense that you can sit back until it is finished (but it is built in "on-site production"). A prefabricated house provider, on the other hand, is someone who completes the entire shell - usually in timber frame construction - in their workshop and assembles and screws it together on your construction site, after which their subcontractors finish the interior.
If I were you, I would now take Katja’s design as a concrete point of comparison, along with the photos of the plot and the drawing of the elevations, and go to several providers with these.
Either they then tell you what it would cost to build this design. Then you continue working with this, asking them the Why-are-you-more-expensive question until the price differences are clarified.
Or they counter with their own building proposal, and you gain more variants to consider. In the process, you already see how clever the guys are.
The worst ones will simply offer some off-the-shelf design with a similar square meterage and say how it costs differently with more or less knee wall.
At the end of this round, you have a bag full of offers and specifications, and a blacklist of duds.