Interesting, I hadn't thought about this one. I just looked it up and didn't realize that they already have an app for blood oxygen, though it isn't continuous: https://support.apple.com/en-us/HT211027
Garmin smart watches have had continuous blood oxygen monitoring for some time now. But measuring at the wrist can never be as accurate as fingertip.
Take their readings with a serious grain of salt. Even with the admonition "not as accurate as fingertip". Their readings are garbage. A random number generator does a better job of reading my SpO2. And I'm particularly pale. It's even worse on people with dark skin.
I see absolutely no benefit in the pulox feature on my garmin watch.
Mine usually reads about 97%, which is probably close to right. But I haven't been able to test it at high altitude yet.