Searching for a prefabricated house provider with land brokerage...

Some providers told me that you first have to conclude a kind of pre-contract on the basis of which plots of land are then offered from a pool.

Please read exactly what is presented to you. The pre-contract certainly aims at the sale of the house subject to the plot and financing. If you then do not take a house from them, although they offered you some lousy plots of land and arranged an overpriced financing, you will certainly have to pay a contractual penalty. So under no circumstances sign anything with several providers; you only want to build one house, not several.

That is simply snake oil...… I would NEVER sign something like that. NEVER! You just have to take a look on [immoscout] - some "offers" have been there for years (!!!) - even though there are NO plots available at that location. That should actually be prohibited. Just don't be fooled - there are enough providers that don't need to do that. The others are pushy sales teams.....

With the difference that he even accepts a bad location.

Exactly: the "equivalent replacement plots" share at most total size and floor area ratio, often only the total size.

Some "offers" have been listed for years (!!!) - although NO plots are available at that location.

The process is roughly like this: the sales partner learns of a plot; a model is then selected from the catalog that would be allowed to be built there (based purely on size and roof shape, nothing else is checked). The plot is not reserved bindingly; the owner may meanwhile sell it onward. An interested party is then offered a "comparable" plot within about 20 km radius, provided the provider has meanwhile found one that is not already gone either. Meanwhile, the example house is offered for sale on real estate platforms as if it could actually be built like that – unlike the developer, the provider actually does not have control over the plot. Before reposting such offers on the platforms, it is apparently often not checked whether the plot underlying the example (or a similar one) would actually be available. This trick would indeed be a textbook example for Ede Zimmermann’s show "Nepper - Schlepper - Bauernfänger" (Swindlers and Hucksters) of blessed memory. The interested party is led to believe by such an offer that they have reached their search goal: the exact location is only revealed upon request, but a specific place name is given. It is therefore a deception – only it is not guaranteed that the plot is already gone, so there is a minimal remaining chance. If you are not a jackpot gambler, you should ignore such offers.