Underground ?? This is bryond is fiction. It has to be underground because we can’t see it. There is no oxygen on the moon. Absolutely zero. There is no water. There are no little green men or cheese. This is made up to get people to increase the national debt to 23. Trillion to make work for geeks. Let Leon musk go mining for moon dust . Not me.
https://www.nasa.gov/mission_pages/Mini-RF/multimedia/feature_ice_like_deposits.html
The reason H2O on Earth’s Moon would be underground is that surface water would boil off in the vacuum & low gravity, ice would sublime, etc. But conditions a few feet down are much different. By your reasoning, the well @ my home must be dry — luckily, conditions 50 ft. down differ from the surface.
As for oxygen, there is lots of oxygen on & in the Moon. Just not in breathable form. It is in the rocks. Look it up.
I agree about the green men and cheese, however. ;-)
You still did not answer my question: If many moons have from some to huge qty’s of H2O in them, why would Earth’s moon not have at least some?