Remove chain guard from bottom bracket What tool do I need to remove the black plastic cup that is holding
the metal holder for my chain guard?

I can rotate the metal part but the cup is not moving.
While googling I could not find anyone that used the same mechanism.
The splined retaining ring is standard for internal bearing cassette type bottom brackets.

You need a bottom bracket tool such as the Park Tool BBT-32. Googling 'bottom bracket tool 20 tooth' will yield other options. You will also need a suitably  large wrench of course.

Be aware that the right hand ring is a left handed thread. You also need to remove the left hand ring first as the right hand ring is integral to the bottom bracket cartridge.

Check out this YouTube video that shows how to remove the rings and bottom bracket cartridge.