Under-sink cabinet

Hmm well, my wife didn't really want that

Okay.
Maybe you could specify what you want.
I think that Ikea is generally a good choice and can often be combined.
If you want exactly matching furniture for very specific sinks, that can be quite expensive among other things and limited in selection.
As soon as you place a washbasin on a countertop (wood, Ikea, etc.), you can do whatever you want underneath; otherwise, it has to fit exactly!

Can you also attach the matching vanity cabinets to all sinks?

No!
If you get this oval design, you can’t attach anything underneath, only place something. So a normal vanity cabinet, but no built-in...

Hmm well, my wife didn’t really want that

What does she want? Sorry, but there are too many sink possibilities for a general answer – and we don’t know what you have in mind or have chosen or what is included in the construction service description.

Thank you for your responses, we were at the bathroom studio yesterday and were told that the washbasin from the standard series is not undermountable, we were now offered a double washbasin with installation for €1300 from their own brand, but find it somewhat expensive.

Then have the sink removed from the offer, buy a part like that at IKEA, and have it installed on site by the craftsmen. Check that beforehand.

Then have the sink taken out of the offer, buy such a part at Ikea and have it installed on-site by the craftsmen. Clarify that beforehand.

Yes, or find a local plumber, the plumbing company did not want to take it on because they had bad experiences with it.

Yes, or I look for a local plumber, the sanitary company didn't want to take it on because they had bad experience with it

Exactly.
Ikea has, for example, Godmorgen etc., which can also be assembled quite well by yourself without any craftsman training. My idea was therefore to place a basin on top, that would simplify it. Maybe you have only seen unattractive implementations, personally I like these much better and you are free in the design.