When does the Apple Watch take a heart rate measurement?
The Cardiogram companion app for the Apple Watch requires WatchOS 6.2. and above. If you no longer see the app on your Apple Watch, please be sure to download the latest version of WatchOS. - How To Update your Apple Watch
By default, an Apple Watch collects heart-rate readings every 3-7 minutes. If you’d like to collect data more frequently (every 5 seconds), you can do so from within the Cardiogram watch app by turning on continuous reading or by starting a workout on your Apple Watch.
Increasing the frequency of when your Apple Watch takes a measurement does decrease your Apple Watch's battery runtime.