Hi
Sedentary indicates that you had been still for at least five minutes before the reading was taken.
Apple Watch takes background heart rate measurements automatically at intervals that vary, when you are still. The time between readings therefore depends on your activity level. Readings could, for example, be taken automatically as frequently as every one, two or three minutes:
Your heart rate. What it means, and where on Apple Watch you’ll find it. - Apple Support
Heart rate is also recorded continuously during workouts when recording them via the built-in Workout app and third-party fitness tracking apps that support the feature:
Work out with your Apple Watch - Apple Support
Heart rate results can be viewed via the Health app on your iPhone:
Use the Health app on your iPhone or iPod touch - Apple Support