Sanitary pipe installation faulty?

I honestly still don't understand how the installation frames are supposed to be properly fixed. Does the drywall go up to the ceiling?

Yes, it goes up to the ceiling and is reinforced with OSB boards. But since I also have to fix them to the floor, it is therefore not possible.

Where the pipes lie on the floor, there must be a drywall with a thickness of 10cm

The pipes are in the drywall. If the studs stand to the left of the pipes, how wide would the niche be?

If I move the drywall further to the left next to the pipes, I only have 80cm left, which is clearly too little!

No, nothing next to the pipes. Next to the pipes come stud elements. These; together with the system elements on the right then form the drywall. The pipes are inside this wall; or should the installation for the washbasins become a half built-in wall?