I read through this patent and this patent is one of the more intriguing ones that I've seen recently, and that's saying alot considering how many we found today.
The mirror is cut out around the sensor, and an EVF is used to "fill in the hole" where the actual super-35 sized hole in the mirror is. The OVF presentation wraps around, or surrounds the camera EVF view so that when looking through the viewfinder, you have both the EVF view in the center, and a larger mirrored OVF view surrounding it.
This gives the cinematographer to see "beyond" the super-35 angle of view that you would be limited to when using a traditional video camera.
Japan Patent Application 2018-56635