TL;DR: mostly future taxpayers.
When borrowed money comes due for a federal government (and assuming the country has its own money framework), the three main options to pay it out are revenues (taxes, fees, and whatnot), new borrowing (rolled over), or new money created in the money supply. (There is at least one more option: default.) Most indebted governments are rolling over an ever-increasing amount of debt, because obviously new revenues are not keeping pace if there is a net deficit and because increasing the money supply is generally expected to cause monetary inflation.
As long as there are buyers for new debt, rolling debt over can be perpetuated. (If spending growth is gradually slowed, revenues can gradually close the gap.) When there aren't enough buyers for new debt, increasing the money supply to buy government debt is politically less painful than dramatically increasing various taxes and fees. Government probably doesn't really "win" when resulting higher inflation erodes the value of the debt it holds because lenders demand higher interest rates on new debt; long term I doubt it is government that is beating big investors at that game. Default is rare.
Not all government revenues are taxes, hence "mostly".