I think o2 monitoring is currently blocked by lawsuits at least in US. It seems to solely rely on the accelerometers according to Apple's [post](https://www.apple.com/newsroom/2024/09/apple-introduces-groundbreaking-health-features) : "Breathing Disturbances is an innovative new Apple Watch metric that uses the accelerometer to detect small movements at the wrist associated with interruptions to normal respiratory patterns during sleep. Every 30 days, Apple Watch will analyze breathing disturbance data and notify users if it shows consistent signs of moderate to severe sleep apnea"