This information comes from:
https://www.cultofmac.com/610268/vo2-max-apple-watch-aerobic-fitness/
To accurately measure your VO2 max, you must wear a face mask. This is the only way to know for sure how much oxygen is going in and out of your lungs.
Since Apple Watch is worn on your wrist, not your face, this is out of the question. So instead, Apple Watch provides a “predicted VO2 max” metric. This is an estimate based on changes in your heart rate during exercise.
The relationship between heart rate and VO2 max is not precise, and it varies from one individual to another. So Apple’s predicted VO2 max may not be very accurate. The estimate is also dependent upon your Apple Watch’s heart rate sensor. And while that’s pretty good, it is not infallible.
Apple Watch may log your VO2 max, but you can’t currently check it on the watch itself. To see your stats, you must use the Health app on your iPhone. Go to Health Data > Activity and scroll down to find it. If you have readings for the current month, they will be highlighted in a big orange box. If you have no VO2 max data, it will be languishing at the bottom of the list, under No Recorded Data.
If you can’t find any VO2 max readings in the Health app, that is probably because you have not yet logged the right kind of workouts with your watch. It can take at least 24 hours of wearing your Apple Watch, followed by several Outdoor Walk, Outdoor Run, or Hiking workouts and passive measurements with your Apple Watch before you receive an initial estimate. It has to be one of those 3 workout types. VO2 max relates to maximal exertion. So you’ll need to log a pretty intensive workout to generate a reading. And you’ll need to sustain it for at least 20 minutes.