The Treasury simply issues the bonds. It isn't analogous to borrowing money, as there is no collateral to put up and no other entity in the economic system needs to have any savings in order for the US Treasury to issue the bonds. The bonds are traded one-to-one for reserves at the maturity dollar value, so this is not borrowing. It is a swap.

Merely having a different purchase value from maturity value is not enough to qualify a bond purchase as borrowing either, because Bond issuing is not restricted by any economic opportunity cost. The amount of Bonds issued is an simply edict by Congress, when it passes a Budget resolution.