For me, a prefab house is a house that I can basically choose from the catalog and
those are model houses - houses designed by architects for GUs, that is general contractors or construction companies, which appeal to a large group of buyers and can therefore be offered at a lower price.
Prefab houses are pre-constructed, mostly from wood material, yes.
The house we want to build should definitely be a solid construction (it probably has to be, since we have our architect...).
Architects can also plan timber frame construction for you. Every house was once planned by an architect.
go to a company like Viebrockhaus etc because the building permit (or preliminary building inquiry) is not as easy here as in a new housing development
What does a new housing development have to do with the discussion now? Anyway: every house building company employs an architect.
An independent architect is good but also charges fees according to HOAI.