How Do You Measure Heart Rate?

Fitness trackers can count your steps in distnt travel. It's increasingly common to measure heart rate on the latter they are not as effective so again, here we go. The more simplistic the measure, the more accurate and valuable the device. No one is railing against using a stop watch for a beginner runner. And it is very accurate heart right? On a chess monitor, maybe it's one step up from time in terms of complexity and a honesty or accuracy of the measurement.