For normal video/audio communication in letterboxes, you generally need the following components:
Camera: A camera that is integrated into or mounted on the letterbox to enable video recording.
Microphone and loudspeaker: A microphone and loudspeaker are required for audio communication, which can also be integrated into the letterbox.
Control unit: A device that controls the communication between the outdoor camera/intercom system and the indoor stations.
Indoor station: A display with loudspeaker inside the house to be able to see and speak to the person at the door.