Optical sensor is how. Used to work on my S10+ using the BP Labs app.
Samsung decided to remove the HR / O² / BP sensor on the S20 Ultra & devices after..
Guess that Samsung wants people to buy Samsung watches, but apparently there's been a problem getting Health Canada's certification.. 🙄
Your SM-G960W does have an O² Sensor. It's the same sensor used for your Heart Rate.
If you open Samsung Health & Go to Settings > Manage Items, you can view your O² measurements there.