How do you receive calls without a sim? Or does it have a dedicated sim with a different number to your regular phone?
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 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.