Hi maddy1023,
Per Use the Health app on your iPhone or iPod touch:
"Your Apple Watch uses your personal information to calculate how many calories you burn and more. You can choose to share this information with other apps and it's kept up to date across all your devices."
Your device will estimate resting calories based on the information you've entered. If you use other devices that share information with the Health app, those records may be taken into account as well
For more help with your Apple Watch or the Fitness app, reach out to Apple Support.
Regards.