Us currency is one of the safest and most secure in the world. International investors gravitate to such currencies when investing. When interest rates in such countries increase the demand does for that currency does as well. You can’t “buy” a CD, Treasury note, etc. or goods and services without converting the foreign currency into US dollars first. This is how exchange rates are determined. If you are in Europe and buy my US product I want to be paid in dollars. International currency transactions of these sorts on a daily basis dwarf the level of volume on Dow Jones by a large margin.
That’s what Russia, China, Iran and some other countries have been trying to do to undermine us. If they aren’t paid for transactions in dollars, they won’t have to buy our treasuries. They are small potatoes compared to us, but they have been making deals using other currencies for awhile now.