From extant currency.
What do the funds used to buy bonds do until repaid?
They back the newly printed currency.
You see, nothing is ever actually created.
Money is created from Commodities like wheat, corn, and metals, all of which are obtained in manners where the input cost is less than the output value with the additional value coming from a process in nature (sun and photosynthesis, or geologic actions for example).
You've clearly never actually done the flow chart, huh?