Is VO2Max measured for indoor runs?

VO2 Max is not measured by your watch. It's estimated.

VO2 is about air capacity of the lungs, and it's measured by applying a mask and measuring the amount of air transfer during exercise. No consumer wearable device measures VO2 Max.

The Apple watch uses what you tell it about your weight and age and other stuff, measures heart rate and distance and time, and then makes a clever calculation to give you an idea of what VO2 Max might be if you were to have it measured. Different devices use different algorithms to give a good guess, but that's what they are, guesses. Apple's is probably one of the better ones, but be careful about taking it too seriously. Just because the watch gives a number to a tenth is no reason to believe that it's meaningful. If you look at charts about VO2 max, they refer to ranges, like 30 to 35, or over 40. They do that because there is no real meaning to more precision than that. The VO2 Max indicator is probably best used as a way of seeing if anything has recently changed.