It's a box filled with IOUs.
Government gets the money, spends most on recipients, &
spends the rest on other things. But it replaces this overage
with "investment" in securities that it itself creates.
https://www.investopedia.com/ask/answers/110614/how-social-security-trust-fund-invested.asp
If a private company treated trust fund money this way,
its management would be in prison for fraud.
Also, federal monetary policy intentionally expands the money
supply faster than economic growth. This causes inflation,
which deflates currency value, thereby lowering federal debt
in real value.
This is also why capital gains taxes are often confiscatory, ie,
we're taxed on the additional less valuable dollars required for
the same economic value of the sold asset.
This is also why liberals oppose (generally) indexing capital
gains to inflation, ie, wealthy people have capital gains, &
they'll benefit from fairness.