Ok cutting to the chase - When the government wants to spend money that they don't have, and they don't want to raise taxes to get it, they sell Treasury Bonds to the Fed - they borrow from the Fed. The Fed loves this, because they can now create money. The Fed creates money to give to the government (from nothing), which then spends it on government contracts, like aid to Ukraine or weapons to support a war. That becomes real money in circulation then as those contractors take that money and pay people and pay suppliers and put some in the bank. This causes inflation. This is how our national debt keeps growing ridiculously and inflation is inevitable.
What's even better for these bankers, is that these government bonds, bought with fake money that they created out of thin air, become assets on the balance sheet that they can leverage at a 10:1 ratio according to fractional banking rules to lend more fake money out and get interest. This is why the big bankers love war.
Now, the question was, what is the mechanism to reduce the money supply?