How Did The Apollo 11 Documentary Get Film Of The Upper Stage Ignition?

I kept getting asked about a sequence from the Apollo 11 documentary showing the ignition of a Saturn V upper Stage, known as the S-IVB, in space. After asking around I tracked it down to Apollo 9, which was needed the full Saturn V for launch but didn't need the upper stage to go to the Moon.

Skylab Launch video was assembled by Retro Space
https://www.youtube.com/channel/UCZJgMAoS30yYoLOYUe_lAXQ

Видео How Did The Apollo 11 Documentary Get Film Of The Upper Stage Ignition? канала Scott Manley