Answer:
Most wearables with heart rate monitors today use a method called photoplethysmography (PPG) to measure heart rate. PPG is a technical term for shining light into the skin and measuring the amount of light that is scattered by blood flow.
Explanation:
sᴀɴᴀ ᴍᴀᴋᴀᴛᴜʟᴏɴɢ