> That by law can only go to fund social security and nothing else.

Eh, that's partly true. The true statement is that social security tax revenue must eventually go to fund social security.

Before it's needed (i.e. while tax revenue exceeds benefits), it has been used to buy US treasuries -- the funds used to do so then appeared for Congress' general use.

You can see the US treasures the social security trust fund is currently holding here: https://www.ssa.gov/OACT/ProgData/investheld.html