The battery compartment door has a tiny lock that has to be pushed aside to enable opening that door.
As a tool to do so Canon added a tiny cam to that plastic plate which is supplied with the camera to shield the viewfinder window in case it will not be covered by the photographer whilst exposing (to avoid stray light entering the meter).
This shield is typiccally missing with used cameras.
I never ever used that shield for opening that door, but just my (short) fingernail. The door opens most easily. It is the first time I learn about broken doors and this all is enigmatic to me.