How to I manage to remove just this part of the floor?
I tried to close this square like if it was a room, then clicked on "remove floor", but then when I remove the wall, the floor comes back. https://i.imgur.com/Gj4vJVS.png
Try closing it off again so it's a room, then try the sledgehammer tool & hover over that floor. Floor SHOULD light up yellow to show what you're going to destroy.