People get all jazzed up about the Fed, but Banks have to play along Basel III.
Not saying a new crack will not break the system in 2020, but the Fed injection is somehow a side twist in this play.
Same goes for ordinary foreign currency transactions. If you want to short negative-yielding Eurozone bonds and buy positive-yielding US treasuries, you borrow the bonds and sell them, receiving EUR in the process. Then you transfer the EUR to a US bank and receive dollars. Then you use the dollars to buy US bonds. Your European counterparty is now long European bonds and short euros. Your U.S. banking partner receives the euros you got from them and gives you currency. You give the currency to the U.S. government and buy the bond. To make the currency transactions net out, the U.S. government then needs to give currency to the bank to replace the reserves you took out to buy the bonds.
(In reality, the Fed doesn't have to make the transactions net out, because it can adjust the money supply. But if it wants to maintain the current interest rate in the face of strong demand for dollars to arbitrage away interest rate differences, it needs to hand out a lot of dollars.)