A few technical points: The Fed did not spend $1.5 trillion. This was not a $1.5 trillion bailout. It did not cost Americans $1.5 trillion. It was not a $1.5 trillion subsidy for hedge funds and the like. It did not use up $1.5 trillion in resources that could have gone to another cause, whether Wall Street bailouts or Medicare for All.
The Fed works in weird ways, but here goes: The central bank announced that it would offer financial firms up to $1.5 trillion in short-term, collateralized loans. A firm can borrow $100 in cash overnight, for example, but only if it gives the Fed $100 in Treasury securities backed by the full faith and credit of the American government, and pays a small amount of interest too. Doing this costs the Fed nothing, and costs the American taxpayer nothing; when all is said and done, the central bank will probably make a small amount of money off the interest payments.
The Fed chose to do this not as a payoff for Wall Street or to calm the stock market. (It has nothing to do with the stock market at all, though equities crashing is in part a sign of the very financial strain the Fed is attempting to soothe.) It did it to help make sure that the market for Treasury bonds continues to function normally. It was not using taxpayer dollars to juice a money-losing industry, but instead acting as an emergency backstop for the markets writ large.