What stays on your device
By default, everything. Your swing videos, the skeleton extracted from them, scores, history, golfer profiles, names, heights and handicaps are stored on your iPhone and are not sent to S-Motion.
You do not need an account to use GolfMotion's core on-device features. The separate coach portal does create a server-side account when a coach chooses verified email or Google sign-in. Sign in with Apple is not currently enabled for the portal.
Optional iCloud sync copies your library between your own devices through your private iCloud account. S-Motion cannot access that library and does not use it for model training.