Are wooden walls possible in a solid house?

Often corners are impractical and not cozy. You also have to be able to furnish the area next to the built-in wardrobe

It's precisely about preventing corners in front of a wall. I think flush pantry closets are brilliant where it fits. Distributing small pantries and closets all over the house brings even more clutter in my opinion, which one actually wants to avoid. And a stylish dresser, heirloom or Ikea, has already created many a great feel-good space.

We Germans plan quite squarely and practically. For this type of chambers/built-in cabinets, artificial niches would rather have to be created. In a new building, you will hardly find an offset to make it flush with it.

We Germans plan quite squarely practical. For this type of chambers/built-in closets, artificial niches would rather have to be created. In a new building, you will hardly find an offset to make them flush with it

You are right, because every corner costs money. So I suggest in my floor plans, if they are next to each other, mutual use for children's rooms and often draw this kind of built-in closets. The planned wardrobes are nothing else. Even my townhouse, built in 1978, had those. However, we Germans like to build "correctly right away," so rather reluctantly use timber frame construction in addition to solid construction. Solid construction costs more again (edge surcharge), and you want to offer the house as cheaply as possible... So: square, practical, good.

I imagine building massive walls for all of that is more complicated and expensive than doing it with wooden walls. Am I wrong?

Whether and how much you are actually "wrong" there, I cannot say precisely without a specific house design; I also do not have a clear idea of what exactly you mean by wooden walls. US-American built-in closet culture is rather unusual in Germany – also for the construction workers. I would simply give each type of wall the most suitable construction method for itself and would always clarify the room concept before such details.