Who is the builder when selling "turnkey" houses?

We are still at the very beginning of our house building plans, have acquired basic knowledge with the help of magazines, books, and the internet, and now have a fundamental question:

Many of the listings on Immoscout, Immowelt, etc., sell a house and land together. Does this mean that the provider acts as a developer, so that you yourself do not have to appear as the builder in front of all the authorities?

Sorry if I asked the question in the wrong forum or if it has already been answered somewhere. I couldn’t find anything else quickly...

Who is the client

Usually, the client is the owner or user of the plot of land on which the construction measure is carried out.

Many houses with land are offered on real estate portals, where the land is purchased by third parties and the house is then to be built by a construction company. The construction company is usually not simultaneously the owner of the land. In such a constellation, the buyer of the land is the client. Everything that the construction company then builds on the land immediately becomes the property of the landowner (big advantage).

If the construction company is at the same time the landowner (developer business), the buyer of the turnkey house to be constructed enters the land register with a priority notice of conveyance. He becomes the owner only when the conveyance is declared. This is the case when the last installment of the purchase price is paid. This is of course a nice leverage for the developer.

So: land bought from third parties, house construction with construction company = client status with all obligations (but good companies usually take the work off your hands)
Land with house from the same company: no client status, the client is the developer, thus no obligations, just pay.

Regards