Apple watch
If I buy a watch that is gps+cellular, do I have to pay a service fee for cellular with my phone plan?
Apple Watch SE
Yes you would, if you decide to activate the cellular service. However, the Watch could still be used for cellular with the paired iPhone nearby. If you wish to use the Watch with cellular by activating the cellular on the Watch, there is a fee. Here in the US it is $10/month. Other countries are on a similar pay structure.