The National Debt is often confused with the (Annual) Defitcit. The deficit is the amount that must be borrowed yearly to pay for government expenses. The national Debt is the accumulated debt of all government borrowings over time that have not been repaid. Almost all of this is financed by bond sales. The United States is having no trouble financing all the bond sales at very low interest rates. Some European countries with high debt problems and long term deficits are forced to pay high interest rates to sell their bonds.
Some of the government debt is backed by loans made directly by the government with borrowings. FHA home loans, USDA farm loans and various other direct loan programs actually are assets that could be entered on the government balance sheet. There is no government balance sheet like a corporation. All the public lands would also be assets. Varous types of bonds are issued to fund these loans. Any bonds, including savings bonds are debt to the government, though they are assets to the bond holder. Government lending is only a small part of the National Debt but many of these loans are 30 or more years and would be shown a government debt well into the future.
Some US elected officials are not worried about the rising US National Debt but the good US debt rating hangs by a thread. The bond rating companies can downgrade US debt as happened last August and eventually, the government would have to pay higher interest or even balance the budget.
The US has the strongest military in the world which helps keep interest rates low even as the US debt takes up more and more of the Gross Domestic Product. The GDP is the value of all the goods and services produced in the country. the problem is at some unknown point where the National debt gets too large, faith in a country's bonds will collapse and a depression will occur. Nobody knows when or how soon.