https://www.apple.com/watch/cellular...e-family-setup

Seems in HK family setup works for CSL, Truphone and 1010... The list in the presentation yesterday was not exhaustive.

https://www.truphone.com/consumer/esim-for-watch/#plans

Truphone costs 29 HKD a month, so more than others. But, that was new to me, it actually works independently of your phone contract!