Lets take this to the extreme: only 2 people remain with capital and AI all the rest are replaced.
Now these two people how do they make money? they pay each other so there is no extra value created thus the amount of money as value symbol remains constant.
But here is an even more interesting question: As their AI can create anything why would they pay each other? So why do they need money?
The money just circulating around is actually more or less how a normal economy works. If you have a two person world where one person makes food and the other makes tools, the money just bounces back and forth between the two people as they trade tools and food. It facilitates trade by acting as an IOU in case the first person doesn't need to trade tools at the exact moment the other needs to trade food.
AI and robotics will one day be able to produce food and tools without human labor. So there could be plenty of wealth created. The question is how do we distribute that wealth when humans aren't needed to make it? We need a new distribution system that isn't based on pay for labor. A lot of people suggest UBI.
UBI is a system for dividing up scarce resources.
For instance: it'd expect money to become near-meaningless, and the economic activity of the trillionaire class will consist mostly of direct extraction and consumption of resources (basically a personal autarky). There may be some barter of things like energy, raw materials, and maybe a small amount of proprietary items.
Given the lack of need to pay labor and the direct control of more resources than they'll ever need, the trillionaire will direct the world enonomy to towards pet projects (e.g. an Elon Musk commanding his robot army to build a giant steel pyramid on the moon in his honor, because why not? It'll be cool!).