The WWDC23 session number 10016 is referring to a way to generate .workout files (timecode 09:43) in both binary and JSON format.
Unfortunately, the API seems to have changed:
CustomWorkoutComposition renamed to CustomWorkout
.dataRepresentation disappeared
Ideally I would like to generate .workout files directly in JSON format but there is no specifications published and no documentation to generate them.
Any idea where I should look?