Functionally, to the people who shorted stocks or sold while it was still high. But more accurately, it goes nowhere; the "market value" isn't an actual number, it's just a shorthand for "how much all the people currently playing the stock market are willing to spend on stocks." They count up all the shares of every stock currently issued, multiply it by the amount people are willing to pay for those shares, and that's the market value. When it goes down, that money doesn't "go anywhere," because it never actually existed to begin with; it was just describing how much confidence people had in the market.

The people who short stock or who sell high are the ones who have played the game so as to benefit when that confidence drops, so they get some actual money out of the deal; but they didn't "make" $1.8T yesterday. That's just how much less confident investors in the American stock market are.