Well yes but that doesn't mean an entire trillion dollars existed first. Valuation goes by the latest trade price, not total purchase price of all stocks.

A million people could all buy a share of something that has a million shares for 1 dollar and then the some person buys one off one of the investors for 1000 dollars and nobody's selling for any less than that. Congratulations, it's now a billion dollar company. Now some people realize it's a bubble and lower their sell price to 10 dollars. A few people buy. It's now a 10 million dollar company. 990 million was wiped off the stock market.

That obviously simplifies it but you get the point.