Can you use an iWatch on an iPhone's cellular plan

The Cellular Apple Watch shares the iPhone's phone number.  This requires the iPhone cellular carrier to support the Apple Watch.  The iPhone carrier charges a monthly fee for this service.  In the U.S. the fee is $10 per month added to your iPhone 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

You will need an iPhone 6s, SE (1 or 2), 7 or newer, running iOS 13 or newer.

The GPS only Apple Watch can do  Everything  the cellular model can do  as long as  your iPhone is  nearby, where nearby is somewhere in the same 3-4 bedroom home (assuming typical wood frame construction).  And of course when the iPhone is with you when you leave the house.

https://support.apple.com/en-us/HT205547

The Cellular Apple Watch allows you to leave your iPhone at home while you go for a walk, run, bike ride, etc...

GPS

Apple Watch Series 5 (GPS) lets you send and receive text messages, answer phone calls and receive notifications when it’s connected to your iPhone via Bluetooth and Wi-Fi. The built-in GPS works independently of your iPhone for distance, pace and route mapping in workouts.

GPS + Cellular

Apple Watch Series 5 (GPS + Cellular) has all the same features, as well as built-in cellular. It gives you the freedom to send and receive text messages, answer phone calls, receive notifications, and stream Apple Music and Apple Podcasts — even when you don’t have your iPhone with you.