Toilet connection is neither on the wall nor on the floor.

So laying a drainpipe is not a big deal?

That depends, among other things, on the further course of the pipe and the thickness of the wall. But based on the picture, no one can answer that for you. With some craftsmanship skills and common sense, one should be able to manage it. In case of doubt, there are companies that make their living doing just that :)

What is hanging there next to the cistern on the left?

What is hanging there next to the cistern on the left?

Toilet paper :D

What is hanging there next to the cistern on the left?

exactly is toilet paper roll

exactly is toilet paper roll

:D now I see it too ;)
On the go, my pictures are always displayed a bit pixelated - so it looked like a heater ...

So laying a drain pipe is not a big deal?

For a plumbing professional, rather not. But, depending on the further course of the pipes, some chiseling work may of course be required to expose the pipes. In my case, the flush tanks were already directly in the wall and not in a pre-wall installation. Nevertheless, everything was removed and replaced with new installations. So many things are possible, but the amount of dirt generated should not be underestimated either. In any case, I am glad that the worst of the dirt is now over. I also had all the pipes running in the vertical supply shafts in the wall replaced from the basement to the ceiling between the ground floor and the upper floor. I am currently not having the bathrooms in the apartment on the upper floor done, but when that is due, the rest can be replaced from above as well.