there is no Social Security trust fund

The U.S. Supreme Court in Helvering v. Davis (1937), ruled that Social Security was not a contributory insurance program, on page 635, speaking of both the employee and employer taxes:
“The proceeds of both taxes are to be paid into the Treasury like other internal revenue taxes generally, and are not earmarked in any way.”

Read it for yourself

Congress can spend your tax money any way they want to. There is no law requiring a “trust fund” or any law requiring them to invest the money, earmark it in any way, or even promise to pay you.

You did not “pay into” Social Security. You paid a tax. FICA is not insurance, it is a tax. You will find it in Chapter 24 of the Internal Revenue Code.