On the iPhone Series 6 Apple added an App to interface to the Apple Watch to measure blood oxygen levels.
On their website they state "Measurements taken with the Blood Oxygen app are not intended for medical use and are only designed for general fitness and wellness purposes."
This approach using dedicated sensors has promise and I hope eventually they will get this technology working and gain FDA and MHRA approval. Even with all Apple's engineering and financial resources, they are finding this hard to achieve.
Generally, we would recommend instead you buy a pulse oximeter, costing around £20, more info here: https://www.digitalhealthcoachuk.net/pulse-oximeters
Link to Apple support page: https://support.apple.com/en-us/HT211027
Link to Apple support page: https://support.apple.com/en-gb/guide/watch/apdaf17aa5ef/watchos
Link to an interesting article by American paramedic Rich Mogull on testing the Apple Watch, who is more positive: https://tidbits.com/2020/10/22/the-paramedics-guide-to-blood-oxygen-and-the-apple-watch-series-6/
Article by the Skeptical Cardiologist' https://www.medpagetoday.com/blogs/skeptical-cardiologist/88729 on the usefulness of Apple health monitoring.
Also some other Apps in the iStore claim to measure SPO2 using the phone's camera
Pulse Oximeter App - or any app using the iPhone camera
"The DigiDoc Pulse Oximeter app is not intended for any kind of medical use. It is only meant to be used by sports professionals or anyone who is interested in checking their blood oxygenation level (SpO2) and heart rate. One can use the app in a wide range of settings, such as running, hiking, between workouts, and a number of other activities."
Personally, I wouldn't trust any App using the camera unless it had completed clinical trials and gained FDA and MHRA ]CE] approvals.