Installing a new bathroom in an old building is overwhelming

A very good day to everyone! Since I have shared an apartment with a friend and now want to move into an area, we are currently in the process of or want to build a bathroom. The connections are partially already laid and a toilet installed. What is currently really giving us a headache is: The floor in the bathroom was partially opened by craftsmen regarding the wastewater pipe and we don't know how to fill this hole there to ultimately lay a neat bathroom PVC... We naturally want to hide the connections on the wall and are considering on the one hand what we can use to insulate there and how to build up the wall so that it doesn't rot. Since one wall is or was built with wood, the question is how to best protect the wood from moisture... And the other wall is simply a classic wall with textured wallpaper and we are considering whether we can paint it over with a special paint or if we have to do it differently... I know these are really many questions and I would be all the more happy if some of you have a good tip for us... since we are not skilled in this area, we would greatly appreciate any help! Then I wish you a nice evening and thank you in advance!

Tear out the wooden cladding on the floor and then install suitable drywall for the bathroom on the wall, tiles or vinyl on the floor.

Advantage: You know that nothing is botched anywhere and everything is sealed.
Disadvantage: It is a bit of effort, but should be doable.