Once upon a time the Treasury had gold (and other real assets). When the Fed was created, the Treasury used their assets as collateral to borrow money from the Fed. The Fed looked at the income and expenses of the Treasury and Congress and government and lent them money in defined increments.
To maintain cashflow, the Fed sold the bonds to the public or institutions. So the Fed was a trading house and made a market on both the bid and the ask. Their inventory came from the Treasury.
Economists and analysts were hired to predict the economy so as to make sound lending decisions. Their only customer to lend to was the Treasury.
Since then, the Treasury lost their collateral, and the Fed kept on lending. The Fed then looked at the future income based on tax revenue and lent them money based on that.
Nowadays, the debt ceiling is the maximum amount Congress allows the Treasury to sell (or the Fed to buy). That should probably be called the yearly inflation amount rather than the debt ceiling.
It is no longer "once upon a time". Today, the Fed conveys nothing but a funny check to the Treasury. Consumers call that "check kiting". Because they are the Fed, no one catches them and they use open market operations to hide the fact that the Fed is creating money to the total amount of the debt ceiling.
So, taxes must make both the interest and principal payments. This has turned it into a slavery system.
The Fed broke their promise. If they were still running their business, they would have stopped lending a long time ago. But. Now they have buyers in the form of the public and institutions who purchase the bonds and create derivatives using them as collateral. Everyone is betting on the future tax revenue. And the fact that the Congress keeps raising the debt ceiling.
The Fed has a fraudulent journal entry. For those who remember accounting, what is the offsetting asset or liability that the Fed credits when they accept the bond asset on their books. Note, if you say cash, is that cash from open market operations?