Does an Apple Watch SE GPS, 40mm Silver Aluminum Case with White Sport Band - Regular require a cellular plan other than the plan on my iPhone? What can / can't the watch do without adding a separate cell plan?

GPS

Apple Watch (GPS only) 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 (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.