Starting point: money can't buy happiness.
So what to do to be happy? Extreme wealth removes most practical goals like buying things or going places and doing things. Not that you can't do them, but it's not a meaningful goal to work towards.
They have to create their own meaning, whatever that is.
A billionaire trying to create purpose for themselves can be boring, or weird. Which one gets media coverage?
Gates Foundation, Zukerberg's fitness craze, MacKenzie Scott's philanthropy, Bezos and Musk's [whateverness] are all just variations on a theme. And like all people, some will be better at it than others.
Note though, that they will do what it takes to stay wealthy because what would they be without that?