The policy subject of selling government bonds refers to the government. The government sells bonds to the public to recover liquidity. When buying bonds, commercial banks reduce their cash reserves to buy bonds, which is equivalent to reducing their reserves and naturally reducing the money they can lend.

National debt is not a monetary policy, but a fiscal policy. As for the sale of government bonds, it cannot be said to be an expansion policy or a contraction policy. When the economy expands, the government can also sell bonds to raise funds (as the China government has always done), and when the economy shrinks, the government can also sell bonds to increase fiscal expenditure (as the United States does).