Felix Salmon explains the mechanics of quantitative easing:
The way that QE works is that the Fed will publish a schedule of how many Treasury bonds it intends to buy and when....What that means is that the New York Fed has a direct line to the biggest banks in the world (Goldman Sachs, Morgan Stanley, Deutsche Bank, etc — 18 in all). And it gets all those banks to compete with each other, either directly or on behalf of their clients, for who will sell the Fed the Treasury bonds it wants at the lowest price.
....The people selling Treasury bonds to the Fed, then, are big banks, who are told in advance exactly how many Treasury bonds the Fed wants to buy. As a result, they’re likely to buy Treasuries ahead of the auction, with the intent of selling them to the Fed at a profit....Once the banks have made that profit, it’ll get paid out in bonuses to the people on the bank’s Treasury desk, with the rest going to their shareholders. We’re not exactly helping the unemployed here.
More detail at the link. And for yet more detail, Felix recommends Shahien Nasiripour's long HuffPo piece about Fed policy and how it works.