There are many tools involved - a tool by itself or in combination with others.
For instance , here's a YT video on how to add wallpaper....
https://www.youtube.com/watch?v=KF9PxZfeqwg
Adding a window to a wall may require the transform tools to resize or distort the window to match the wall perspective. And then, match the colors and exposure to blend in the new element into the scene. This will require adjustment layers and applying a clipping mask. Clipping masks are used very regularly in PS and when applied, the adjustment layer effect is visible only where the new window is visible. We can then adjust the colors to match without affecting the rest of the photo. And then there's the matter of adding an outside scene in the window....
Same could also apply to floors unless you require to change or add a texture. This is where using the displacement Filter will come in handy.
There's an old thread here that touched on "re-furbishing" a living room... which I can't find right now....
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It would help if you can post an image we can work on so we can give it a thought of what to give you in terms of basic, simplified steps... but bear in mind, basic, simplified steps may not be enough to create a believable refurbishment....