What if the niche doesn't open?
The one with the traffic lights on the table just turns the third one green (that is, turns on the switch of the third light);
Why can I still see the small ingot when the No.2 light is turned off?
That ingot is luminous, so it will be very dark when the lights are turned off, because each switch controls one light, and the second one is the light in front of the ingot.