Galaxy Watch: Samsung makes no clear statement either way on whether the Galaxy Watch sensors are medical grade or not, neither on their English web page nor in the user manual. Which is disappointing. Link to Samsung site: https://www.samsung.com/au/support/mobile-devices/measure-blood-oxygen-levels/
Samsung Phones: For quite a few years Samsung have been trying to develop a working blood oxygen monitor as part of their Galaxy phone range, and this data is accessible via the Samsung Health App. However, the sensors are not medical-grade yet, and they recently hid the blood oxygen monitoring feature within the Stress monitor in the Samsung Health App.
Generally, we would recommend instead you buy a pulse oximeter, costing around £20, more info here: https://www.digitalhealthcoachuk.net/pulse-oximeters
If you want to know how to find your blood oxygen reading in the Samsung Health App, follow this link: https://www.sammobile.com/news/how-to-measure-blood-oxygen-saturation-spo2-galaxy-device/