So I purchased an Apple Watch Ultra for my partner on my account and paying for cellular service. How do I connect the watch to his iPhone without him being charged for anything?
In case you are paying the cellular plan for your partner on his iPhone, he can set up the watch with his Apple Account after you removed the watch from your Apple Account, if you already paired the watch to your iPhone.
The cellular plan for the Apple Watch has to be supported by the iPhone carrier.
Apple Watch - Carriers - Apple
If you did not set up and pair the watch to another iPhone at this time, and you used your Apple Account to buy the watch online from Apple, he can set up the Ultra with his Apple Account as well.
Set up your Apple Watch - Apple Support
Buying a watch on my account for someone else to use on their own iPhone
