The U.S. Federal Reserve Note (AKA: U.S. Dollar) is the dominate reserve currency. There are many reasons for this. But many of these reasons are now gone or nearly gone.

Here is how the reserves of the world are generally distributed.

Here is an example of how the USD was used in trade with China & Russia.

Although recently both China & Russia have made bilateral trade agreements & currency swap agreements that by-pass the USD in their trade.