A central bank buys Treasuries to hold reserves; a fund, a bank or a stablecoin issuer buys them for yield or because a law requires it. The two behave differently in a crisis and are counted separately by the Treasury.
Since 2021 the official side has been a net seller and the private side has absorbed the issuance. That is the structural fact behind every promise financed by new borrowing: the buyer is increasingly private money, and increasingly money that must hold bills by statute.
When a politician promises a payout, this is the ledger line that says who lends for it.