I found the below description of the 1.5T injection by the Fed. Comparing this to the purchase of military equipment or the idea of clearing student loans should be avoided.
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What the Fed has done here is gone to banks and said “I will buy $500 billion in government bonds if you agree to buy them back in X days for $500.0001 billion dollars”. The banks agree to this in order to ensure they have enough cash on hand to loan to other banks and businesses. This is kind of a loan in the sense that the Fed is temporarily giving banks money to be repaid for a profit, but the key difference is that the Fed holds the bonds as collateral in the meantime. This means there is little to no risk of default on these loans since the Fed can turn around and sell the bonds to recoup any losses. The key point here is that this means this isn’t really a stack of cash sitting around that could be redirected. The fundamental mechanism here is a temporary injection of liquid cash in exchange for safe collateral. That isn’t the case with other suggestions floated around like using this money for a temporary UBI, or canceling student loan debt.
It may be surprising to know that these types of repurchase agreements happen every day as a normal mechanism to keep the economy moving and regulated. Usually, they are for smaller amounts (I think around $100 billion each day). What the Fed has done here is increased that amount to $500 billion over three days, which is why you alternatively see headlines saying they are injecting $500 billion or $1.5 trillion into the economy. The reason this can happen without much debate is that it is to a large part ‘baked in’ to normal operation - the only thing that has changed is the daily amount, and even then temporarily.