FICA and Self-Employment revenues, by law, fund the Social Security Trust Fund. The fund is invested in the National Debt. Such investments are interest bearing and known as "Treasury Securities", or "Treasuries" for short. They are equivalent to the Treasuries (bills, notes, and bonds) purchased at U.S.Treasury auctions by other investors such as banks, wealthy individuals, and foreign governments.
That's why we owe so much money e.g. to China, Japan, Saudi Arabia, who buy Treasuries because they have no better place to put their surplus dollars accumulated by selling things that are consumed in the USA. So China funds our wars too, and we owe China the same way we owe the SSTF.
And we pay interest on Treasuries. e.g. if you own a Treasury Bond you get paid interest every 6 mos. until the Treasury bond matures. Before modern electronics bond holders had to clip coupons off the security to collect the interest. The expression "coupon clipper" was used, derisively, to describe someone rich enough not to have to work for a living.
Because Treasuries are loans, they have to be paid back like any other loan (i.e. redeemed) at the end of their designated time periods. If they're not redeemed, they're rolled over into a brand new Treasury. The quality of a government is judged by the faith investors have in its debt as a place to park money.
I hope that, if future events cause panic selling or redemption of Treasuries, the SSTF will be honored as much as other Debt holders are honored in such a situation. So far the Treasury has been able to auction off its debt whenever it wants or needs to. Buyers magically appear when needed at the auctions. Let's hope that condition persists, and any security holder is redeemed at maturity, or can sell his/her security whenever he/she needs the money. (The amount of interest is determined by the selling prices that occur at the auctions.)
The problem with Social Security is actuarial. i.e. all the bonds in the SSTF will eventually not be enough to cover the expected number of retirees, unless the size of the SSTF is increased. That would be easy to do, if everyone in Congress wanted it to happen. Also, sadly, some in Congress want to stiff all the National Debt holders. But the buyers keep magically appearing at the debt auctions.