Social security is a big one actually. Any time you hear about the feds raiding social security, they’re buying bonds with the funds and using the cash reserves.
It’s generally safer long term to hold cash in bonds and the government needs liquid cash all the time, so it makes sense. As they collect social security taxes, they issue bonds instead and spend the cash, and pay the bonds back over time with interest in a steady pace.
$7.2 Trillion is intergovernmental. One Federal agency giving cash to the general fund in exchange for a bond.
Beyond that, basically any entity holding bonds. $26.2 Trillion is everyone else.
Every remaining pension system (the federal government retirement system, pretty much every state and local government retirement system, and all private companies that still offer a pension) all invest in bonds for long term fund security and the interest payouts are integral to managing those funds. They also sell off bonds as needed to fill gaps in their funds.
Every 401k, every IRA, every college endowment, etc.
Then yeah plenty of billionaires and huge companies that park money in bonds.
About 1/3 is held by other governments. Japan being the single largest holder. China follows closely in second.