How to create a realistic Shadow Effect

I too watched the video. I think that the presenter(s) of the video wanted to create some fall off of luminance on the folder. When we look at the blank folder at the start of the video we see that there is some subtle shading most notable at the back cover, near the crease between front and back covers. By duplicating the original background folder and converting the layer blend mode to multiply, the presenters were attempting to create that shading on the new covers.

I think the shading created by that multiply layer is too subtle to be easily seen in the video. What I’ve done in the past is to create a black to transparent gradient (most often a linear gradient), set the gradient layer to multiply and then clip the gradient layer to the appropriate smart object layer. Finally bringing the grad layer’s opacity up from zero to where it is just enough.

Hopefully, this screen shot will give you the idea. I’ve exaggerated the darkness of the gradient’s shading to be more easily seen.