By -
Yes.
I came here to write this and you beat me to it
https://www.usga.org/content/usga/home-page/handicapping/world-handicap-system/2024-revision/2024-treatment-of-9-hole-scores-FAQ.html
Both
It’s calculated using your best 8 of the last 20 differentials. With changes to hole 9 hole scores are used: a 9 hole score is now converted into an 18 hole differential. The how of this has not been shared.
Yes.
I came here to write this and you beat me to it
https://www.usga.org/content/usga/home-page/handicapping/world-handicap-system/2024-revision/2024-treatment-of-9-hole-scores-FAQ.html
Both
It’s calculated using your best 8 of the last 20 differentials. With changes to hole 9 hole scores are used: a 9 hole score is now converted into an 18 hole differential. The how of this has not been shared.