Which technological invention allowed millions of people in America and around the world to witness the first man to walk on the moon in 1969?

Television

Explanation:

Since the race to the moon were a huge part of competition between the Western block and Eastern Block led by the soviet union , television was very important to provide a visual representation to the event.

At that time, Neil's Armstrong journey was covered by various medias and broadcasted through televisions. It is estimated that around 53 million households watched the even live on their television.

When we look at the number worldwide, around 650 million viewers  witnessed the first step that mankind ever took to the moon.