Why is _no one allowed_ to install my washbasins?

The plumber also said something about warranty claims, but I didn’t fully understand it.

You provide the items, he installs them. Then he takes over the complete warranty for the next few years, even if, for example, you bought defective items. The risk shifts from you to him.

Washbasins ordered from the internet involve transport across Germany/Europe with all risks (microcracks, etc).
Mounting over tile edges always results in stresses in the washbasin due to the necessary torques, you lean on it, support yourself, always stresses. No one takes that risk.
The only way in my opinion, 10-100€ under the table or a friend for free.
Advantage: if one breaks, the drill holes fit again on the replacement washbasin.

Ah okay, that really sheds a lot of light on the situation. The craftsman would basically be liable if I have damage to my [WASCHTISCH]? Wow, that makes a lot more sense to me now. I wouldn’t do it either if I were them. I thought it was about water damage in the wall and stuff like that. That’s why I didn’t understand the objections, I mean, they have to be liable for that anyway in new construction. But for the foreign material as well, that’s obviously a different matter. Then we probably have to decide now whether we want to further develop our craftsmanship skills or return the purchased components and take another look in the general contractor’s showroom to see if we can find something we can more or less live with.

This is not rocket science either.

The connections should have been prepared by the plumber.

Then you need to have the mounting for the table and then screw it tight.

Just make sure not to tighten it too much because of tension.

This is not rocket science. The connections should have already been prepared by the plumber. Then you need to have the mounting for the table and then screw it firmly. Just be careful not to tighten it too much because of tension.

That is exactly what worries me. Especially the wall mounting, on tiles, in our own house, by people who have never mounted anything on a wall in their lives. :/ I can't imagine that the first attempt will be straight and good. Also, I have to cut holes for the drain and tap hole in our solid wood panels with a thickness of 4.5cm. I lack both experience AND tools for that.

For the holes, look for a hole saw. You simply insert it into the drill. Buy a board of the same thickness at the hardware store and practice. Get a tile drill bit and practice on an old tile. Watch tutorial videos on the well-known video platform and search for instructions on the hardware store websites.