I have a bachelors in economics and your pretty close except you’ve got realize that all money is like this. If you’ve got a us greenback in your pocket and if I decide it worthless to me, and you want something from me then all you’ve got is a small piece of art.
All value is just agreements between people and money is just a method of keeping track of favours owed that we worked out thousands of years ago. In the example of a stock, all a stock is, is an agreement between the company that issues it and its holder, that the company will do you a favour (give you some money, a dividend) some time in the future, and keep doing it every time they have some extra favours to hand out.
I may decide that the promise of those future favours are worth doing a lot of favours right now or only a few.
Right now everyone who buys and sells stocks doesn’t think a lot of companies aren’t going to be able to a lot of favours in the future so the value in the stock market has gone down. But that doesn’t mean a trillion dollars has disappeared it just means that the offer of money is lower