Make foreground object semi transparent

I'm really new to this but I think the software looks amazing! There is a lot of things I still do not understand about the concepts in there so my question may seem dumb but I will ask it anyway...

I would like to build a lens that would make the person semi-transparent. We would see the person and it's background at the same time.

Of course the person would have to move so that the background could be recomposed and then we would see the person in semi transparency on top of this recomposed background.

Think of it like somebody is doing a painting and we can see both the painter and the painting at the same time.

Is something similar already done?

Any recommendation on the techniques I would use to achieve that? ( I'm already familiar with the basics of segmentation so you can skip the obvious.)

Thanks