The Watch is not marketed by Apple as a device to detect or monitor Sleep Apnea.
The watch has sleep apps that can monitor sleep patterns. The watch can monitor respiration. There are 3rd party apps that claim to be useful in detecting and controlling snoring.
If the data from an Watch is useful to your doctor who has diagnosed your Sleep Apnea, then by all means use the watch in that manner.
M1 Mac mini, macOS 12.1; Watch, series 5 GPS/cellular, watchOS 8.3; iPad Pro 2nd Gen, iPadOS 15.2; iPad Air 2, iPadOS 15.2; iPhone 12 Pro, iOS 15.2; TV 4th Gen, tvOS 15.2; HomePods, audioOS 15.2 and Airport Express w/ Airplay 2
M1 Mac mini, macOS 12.1; Watch, series 5 GPS/cellular, watchOS 8.3; iPad Pro 2nd Gen, iPadOS 15.2; iPad Air 2, iPadOS 15.2; iPhone 12 Pro, iOS 15.2; TV 4th Gen, tvOS 15.2; HomePods, audioOS 15.2 and Airport Express w/ Airplay 2