How much more does one pay for "fixed price"?

Don't worry, no house provider ties up capital to buy a plot of land away from you. The story goes roughly like this: 0. potential buyers hardly respond to simple advertisements, so they wait for them where they will turn up on their own (he has to come through this narrow alley...) - so they "advertise" in the form of supposed "land advertisements." 1. someone offers a plot of land; 2. a salesperson contacts this seller and worm their permission to offer the land to their house buyers - this costs the seller nothing, so they usually accept without resistance. 3. now they check what the development plan allows there - result e.g. house model 135. 4. The house provider then places a house sales ad, "Model 135 from EUR 348,000 (construction starting very soon) near 86420 Grundstücksdorf, more details about the location on request." At no point were they a binding exclusive broker, nor did they ever own the land. You have to add the land price on top and of course negotiate with the landowner. Who that actually was, you will never find out if they have already sold their land. Then they look for replacement plots for you, of which the Easter Bunny could tell his grandmother that they are equivalent to the supposedly concrete offer used. If I were king of Germany, such short sellers would be struck by lightning while shitting.

He last did that here: - there is a small bag with links.

2. If a house (e.g. single-family house, 125m², 2 full floors, 4 - 5 rooms) is listed for €200,000 on the website of company XY

Regarding your answer to 1.: On none of the sites I visited was anything mentioned about it. So it neither said that the plot was included in the price, nor that it was not included. Therefore, I thought the experienced homebuilders here might perhaps report how it was for them.

Most were presumably bought from construction companies,

Assuming it belongs to a construction company,

No, the companies do not necessarily buy up the plots, but try to lure you with non-existent plots to then offer you a worse one. There are already threads about this here, maybe can help further

It's not that bad. Most general contractors or construction companies are reputable, they just use the opportunities the market offers.