The Apple Watch has had a cellular option since 2017 with a built in eSIM.
There is a separate number as far as billing. But the cellular carrier pairs your cellular watch with an iPhone on the same network. So calls come in and go out as if they were coming from your iPhone’s number. Whenever someone calls your phone - they both ring.
If your phone is on and close enough to your watch so they are connected via Bluetooth (or WiFi?), all data and voice communication is relayed through your phone to save battery life.
If your watch isn’t connected to your phone, the Watch uses cellular.
Roaming support for Apple Watches was just announced in 2022 and is still not ubiquitous
As far as I know, the watch’s eSIM technically has its own phone number, but the mobile provider does some backend stuff to make it look like it is using your iPhone’s phone number.
We got 3 cellular watches for our kids and set them up on one parent phone using the family connection thing. Each watch used only the phone number assigned to it, for both phone calls and messaging. Maybe they do things differently if you set it up in that way.
As an only device for kids, it was about 80% there and over the course of a few years Apple made 0 progress, so we gave up on it as a concept. Only 1 kid still wears the watch; the other two are e-waste.
There is a separate number as far as billing. But the cellular carrier pairs your cellular watch with an iPhone on the same network. So calls come in and go out as if they were coming from your iPhone’s number. Whenever someone calls your phone - they both ring.
If your phone is on and close enough to your watch so they are connected via Bluetooth (or WiFi?), all data and voice communication is relayed through your phone to save battery life.
If your watch isn’t connected to your phone, the Watch uses cellular.
Roaming support for Apple Watches was just announced in 2022 and is still not ubiquitous