Clarification on singing the Kyrie eleison, Christe eleison, Kyrie eleison

When grabbing the clips I found a few more short Kyries, so looks like there's more to this puzzle, but the info in the video has kept me going so far.
The images of the Little Office and Book of hours are from the Internet Archive: https://archive.org
You can find lots of beautiful manuscripts searching for Book of Hours and Use of Rome and things like that.