About why the dollar has been the world currency for the past few decades (during which EU consolidated into a huge market with a single currency, and China became either the first or second country by economic size): the world's currency needs a country that runs huge trade deficits [0]. China is a nation of savers so their instinct is to not run trade deficits. Europe is in a similar position. US - not so much. I would not expect the dollar to lose this status any time soon.

[0] https://www.dunham.com/FA/Blog/Posts/triffins-dilemma?utm_ca...