Has the Xbox Original and Xbox 360 games that you can download on Xbox One have the graphics or anything improved or?

Are they exactly like if I played them on the corresponding device?

They are not overhauled because they are not remastered.

That depends on the game. Mostly the resolution was adjusted or the frame rate worked.

That is not completly correct. See my answer.

The first Red Death Redemption got a 4k resolution on the One X or a KOTOR is now running in Full HD. Both games also run much smoother.