Do watches have a sim card

Yes. GPS+Cellular Apple Watches have a type of sim card called an eSIM. This is an emdedded sim card, that cannot be removed, but can be set up by supported carriers with their cellular data service.

Note, that only GPS+Cellular Apple watches have an eSIM. GPS-only Apple wacthes do not, and cannot be made to have one after purchase.

A cellular Apple Watch shares the iPhone's phone number. Carriers that support the Apple Watch charge a monthly fee to share the phone number. The fee in the U.S. is $10 per month added to the iPhone's cellular bill.

iPhone Cellular Carriers that support the Apple Watch, which models, the radio frequencies each model uses, and the country where they offer service:

https://www.apple.com/watch/cellular

no they have https://en.wikipedia.org/wiki/ESIM