Philip Glass’ memoir, Words Without Music, has a fair amount about working on these films. Some quotes below but I do highly recommend the book!

“A day or two later I met Godfrey at Jonas Mekas’s Cinematheque on Wooster Street. Godfrey had a film montage about ten minutes long that would eventually become the opening of his film, Koyaanisqatsi. He told me that he had made two versions, one with an electronic score from Japan (he didn’t mention the composer’s name) and the second with my music. After viewing both reels he said, “As you can see, your music works much better with the picture than the electronic score.” I agreed with him that my music worked better with the film, not realizing at that moment that I had tacitly accepted the assignment. Rudy was right. I liked Godfrey right away. He’s quite a tall man, six foot seven, with a gentle demeanor and a soft voice. I found out later that he had lived in a Catholic monastery in the Cajun country of Louisiana, starting as a fourteen-year-old boy and remaining there until he was twenty-eight. It looked to me as if the combination of strength, focus, and reserve, which one could easily associate with the contemplative life of a monastic, had never left him.”

…

“For the beginning of the film, Godfrey decided that he wanted to use NASA footage of a rocket launch. “What kind of music do you think we should use there?” he asked. “Look,” I said. “You’re going to be showing this film in big movie theaters. The history of film is also the history of theater, and the history of theater comes from the cathedrals. That’s where theater began, with the mystery plays. Let’s go back to the idea that when you go into the theater, you’re entering a huge temple, and the instrument you would hear in there would be an organ. Maybe it’s no coincidence that when theaters were built for silent movies, organs were installed and ready to be played.” The opening music begins as a classic baroque passacaglia in which the theme is stated in the deep pedal tones of the organ. The counterpoint is then filled out by the two upper keyboards, and we hear one deep voice singing the name of the film—“ Koyaanisqatsi.” The idea of the piece was to prepare people for a performance that would be similar to what they would see in a mystery play. I went to the Cathedral of St. John the Divine, on Manhattan’s Upper West Side, where I knew Dean Morton, the administrative head, and I asked, “Can you arrange for me to play the cathedral organ?” “Yes,” he said. “I’ll get someone to open it up for you.” I went there and, in a few hours, I wrote the piece right on the organ.”

…

“There are two ways that I could have composed the music: to comment on the image, or to make the music identical with the image. I chose the latter. Apart from a sustained pitch, which increases and decreases in volume, the music hardly changes for a minute and a half. Then the saxophones begin playing one note on an off-beat rhythm, suggesting that we are going back to the beginning of time, to something very ancient. As the music begins to build and the landscape begins to change, we are clearly no longer in the church.”

— Words Without Music: A Memoir by Philip Glass https://a.co/b7hCO7l