Can anyone help me how to get this effect as simple as possible?

B&W background track.
JC on foreground in collor.
- with rounded cropped screen
- and slightly dissolved in the background

Try rotoscoping with a lot of feathering or an alpha shape (ellipse) with the appropriate Min and Max values and a good Transition Width setting.
Alpha shape is probably easier for the roundness, while the rotoscope effect has a better feather setting.

Thanks Bernd. Gonna try that.