In the world of central banking, this muted statement qualified as seriously radical. The new policy didn’t put a number on the bonds the Fed would buy, a tacit nod to the idea that it could buy unlimited amounts.

Then, within days, President Trump signed a $2 trillion economic rescue package into law. That package alone equaled about half of what the federal government spent in all of 2019.

Nobody knows exactly how much the relief bill will add to the national debt; it was cobbled together so quickly that there wasn’t time for the Congressional Budget Office to carry out an analysis, though one is expected to be published this week. But the Fed will be the biggest buyer by far of the bonds the government will sell to fund this spending.

Goldman Sachs analysts estimate the Fed will buy $2.4 trillion in Treasury securities as part of its recently reintroduced bond-buying programs. Economists at Morgan Stanley put the number at around $2.5 trillion in 2020 alone, rising to as much as $3 trillion for the entirety of the bond-buying program.

But the Fed isn’t an ordinary bondholder: By law, it has to pay its profits to the Treasury.

That means when the Treasury makes payments on bonds held by the Fed — either paying interest or paying it off at maturity — almost all the money eventually moves back to the Treasury.

When a government bond is involved, the cash moves from one government pocket to another.

“Once the central bank buys them, it’s as if the Treasury never issued them in the first place,” said Dr. Kelton, who was an adviser to Senator Bernie Sanders’s 2016 presidential campaign. “For all intents and purposes, they’re retired.”

The CARES Act borrowing is, in many ways, the natural result of an evolution that began with the 2008 financial crisis. The bond-buying programs that central banks undertook around the world helped ensure low-cost financing for governments running giant deficits as policymakers contended with a deep recession and a prolonged period of lackluster growth. And despite constant, high-decibel warnings that such an approach would surely ignite a surge of inflation, it never happened.