Help needed trying to find out

Please explain the intended operation.
There are three components whose functions need simple explanations:

  1. What does the IR LED receive and what is supposed to happen when it receives?

  2. What is expected to happen when you push the momentary trigger?

  3. What is the purpose of the relay? Usually a relay has two wires that energize a coil and switches the connection state of 2 or 3 wires or a multiple thereof. Here I only see what I currently assume is the coil connection.

Please help us understand the function better.