Hey Nyawi4,
It appears that you have some questions about how your Apple Watch SE monitors your heart rate, and we're glad to provide assistance.
Monitor your heart rate with Apple Watch - Apple Support -- Please review this information provided by Apple, it explains how your heart rate is determined. To get a more accurate measure of your current heart rate, you can also touch the Digital Crown.
Get the most accurate measurements using your Apple Watch - Apple Support -- To help in getting the most accurate measurements, make sure your personal information is up to date, make sure that Wrist Detection is on, and make sure the band is not too tight or loose on your wrist. Especially, review the information under "Get the most accurate heart rate measurement" and "What else affects your heart rate reading".
If you're still concerned about your heart rate, we recommend consulting with your doctor.
All the best.