If Humans Had Found A way To Live Under Water, How Would They Be Able To Survive Living Under Water?

I'm currently writing a sci-fi fantasy fiction book, where a certain human civilizations live underwater.

But I'm not sore how I would do that

Here's the part

Thirty-five years earlier, humans had found a way to live underwater, they had built cities down there, they even revamp what they called “New Atlantis”, along with human civilizations, there were other types of civilizations; like aliens and mermaids, and civilizations yet to be discovered.

Both the Hope Force and the Magic Elites had people living underwater and not just in the Bermuda Triangle, there were other subaquatic cities all throughout the ocean, and they weren’t just for the high class, it was for everyone including regular citizens of all societies.

Note: The Hope Force are protagonists and the Magic Elites are the antagonists

 

A common trope of old science fiction novels especially from authors like Jules Verne is how they go into depth explaining how a situation would not only be achievable but sustainable.

It's been a while since I read the book and watched the movie for it but 20,000 Leagues Under the Seas has a brief explanation of how the inhabitants of the submarine would be able to survive for long periods without dependency on outside support including while underwater.

https://en.wikipedia.org/wiki/Twenty_Thousand_Leagues_Under_the_Seas

Beyond that it's important to remember that there aren't just fish in the ocean. Whales are mammals and thus a potential source of milk. It's difficult to find information on with basic searches but apparently a blue whale can produce a couple hundred liters a day.

That said how practical it would be is another matter as it's extremely fatty to the point of being almost a solid, is high in protein, and apparently has a weird taste. You would definitely have to heavily process it.

https://northamericannature.com/how-do-whales-breastfeed-underwater/