Excellent YouTube channel showing some history of NASA space flight. I’ve been watching the videos on the Apollo missions. I’ve known some things about Missions 1 and 11-17, but I didn’t know much about 2-10 (I don’t see anything about 1-3 in the channel). Each mission tested something new. Watching these I get a sense of how much preparation went into the later missions. You can also get a sense of this by checking out the Apollo missions Wikipedia page, which covers the many earlier unmanned tests.
What really surprised me was the rapid progress. Manned Missions 7-11 were all launched in the span of less than a year, from Oct. 1968 to July 1969. In 7-10 all the systems, procedures, and maneuvers were tested. Problems were found and fixed for Mission 11 in the span of months. Amazing, considering that the total of each launch vehicle was made of millions of parts.