While once upon a time it was America’s gold reserves, I remember reading recently that the US and Saudi’s did a deal - I think I the 70s whereby oil can only be bought in USD. So globally you need dollars to by oil on the global market. Needless to say, Russia and China are trying to change that - but it is this arrangement that makes dollars imperative as a global trading currency.

Besides that imperative though, the power of the US economy gives people across the world more confidence in the US dollar than their own currencies - which is why China holds trillions of US Tbills. Those Tbills give credibility to their own currency. (At least according to what I’ve read - it’s not my professional discipline).