This is how. These mirrors shine light right back to any source. Astronomer's have been shining laser light on this for years. Those mirrors were put there by the crew of Apollo 11.
BS. You don’t think we can bounce signals off the moon and calculate its relative distance without an alleged mirror? Why fake all the pictures and videos with all subsequent missions only going less than 350 miles if we really did land on the moon (238,000 miles away)