It only needs to be paired to an iOS device for initial setup and configuration. A watch with cellular can operate independently of the iOS device after setup and configuration.

It's been a year or so, but some carriers refused to sell me a watch only plan, it had to be a new cell plan for a phone too. (And pairing with an existing phone meant it had that cell's number, not a watch only one.)

The iPhone ecosystem has "family watches" now.