The government participates in the bond market by selling money. The Fed acting on a variety of data inputs sets the pricing of money which then is sold to banks. Investors wanting to play lender to the government purchase T bills or other government bonds in various flavors for different durations.
American government bonds are backed the country’s ability to tax its citizens. In a pinch, one could imagine Obama sending military troops with guns to the homes of rich people forcing them to pay the various obligations he has created. Current yields on 30 year government bonds are about 3.8%.