Whenever possible all Apple Watch models will try to use low energy Bluetooth to the iPhone and use the iPhone's larger battery, and better radios & antenna.
A GPS only model does not have the option of using cellular, as it does not include the cellular radio circuits.
A cellular model will use its cellular radio when it can not use a lower power option, such as your nearby iPhone over Bluetooth.
Some cellular features still require an iPhone, such as SMS and MMS messaging.
GPS vs Cellular (make sure you read the footnotes carefully).
https://support.apple.com/en-us/HT205547
https://www.imore.com/apple-watch-cellular-vs-gps-whats-difference