It doesn't tell banks to buy Treasuries.
The Fed then pays for the bonds buy issuing a line of credit for them.
The Fed credits the account of the Primary Dealers they buy from.
Regardless, all of those US Treasuries all end up on the Feds balance sheet anyway.
At this point, $45 billion per month, not all the Treasuries issued each month.
That's OK then. I was worried for a minute.