Who gets the interest on the national debt?

short answer: the interest is paid to folks who buy treasurey bills and gvmt bonds.

in theory some is going to social security fund, but that's just paying interest to ourselves, so it's funny money.

probably a google search of tbill and gvmt bond buyers will show the folks we send dollars to. Saudi Arabia (our friend and ally), and China.

The US will be in trouble if they don't roll them over and want cash.