Generally speaking, a country needs a lot of economic activities to develop, and these economic activities all need money. If the money in the hands of the government is not enough to carry out these economic activities, then borrowing money is an act of the state, and the debt generated is national debt.
To sum up:
National debt is a sovereign-backed debt issued by the state, and anyone can buy it as long as they agree to the agreed terms. The main terms of national debt include term, interest rate, interest payment time, etc. The publicly issued national debt is generally anonymous.
US Treasury bonds are issued by the US government and are open to investors all over the world. Any country and individual can buy it.