For a close-up view, move the mouse cursor to the top right in the pic and select XLarge or greater.
The bottom edge of the mirror glass with the rotatable black plastic ring and 4 notches.
Note the plastic pin extending downwards between the 1st and 2nd notch from the left.
As the ring is rotated into the locked position, that plastic pin rides up a ramp and then drops into a groove visible just under the 3rd and 4th notch from the left.
Of course this ring is rotated only once the mirror is properly in position with the 4 pins mated to the motor housing to locate it correctly. A small screwdriver or wooden coffee stir stick can be used to rotate the black plastic ring into the locked position.
To remove the mirror glass, the black plastic ring must be pushed in towards the rear of the mirror by about 4 mm to have the plastic pin clear the groove it sits in while in the locked position and then rotated.