How was the picture in the mirror taken in the movie?

There are many ways. The easiest way is to find a good angle and avoid your own shadow. Put a piece of transparent glass on the other side of the camera and design it as needed. When editing in the future, remove unnecessary ones. I don't think it is necessary to wait for the fourth floor. If you need the shadow of the protagonist in the mirror, then there is no need to make the contents in the mirror according to your method.