Here’s how the system works. The Treasury determines the amount of money it needs to pay out for all government liabilities, including the interest on maturing bonds, as defined by law.
They sell bonds to the Federal Reserve money center banks. The central bank (Fed) deposits funds in the accounts of the member banks sufficient to buy all the bonds. The member banks deposit the funds in the Treasury account.
They then auction the bonds to other banks at a slightly higher price, allowing them to make a profit. The non-member banks offer them to anyone worldwide with cash who needs to park their money with no risk.
For accounting purposes, outstanding bonds are classified as debt. This is a fiction. They could buy back the bonds at any time by requesting the funds from the Federal Reserve to do so.
As long as the Treasury only pays in dollars and never borrows currency from another country, inflation is determined by the economic equilibrium between supply and demand, interest rates, and taxation.
The decision to provide retired seniors with a guaranteed income is a political and moral one, not an economic one.