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Why does international oil trade primarily happen in dollars then even when USA is neither of those countries nor is it the biggest trade partner for either of those countries?Most international trade happens in dollars. This is due in part to competence: the Fed had electronic payments in 1915 [1]. In part to geopolitics: America was the only advanced economy not bombed into oblivion after WWII, an advantage it used to create the Bretton Woods system [2]. And in part to practicality: there aren't many freely-convertible currencies issued by big, stable countries.
> what industrial base?
The U.S. is a massive manufacturer and manufactured-goods exporter [3]. (Number 2 is Europe, another reserve currency issuer. Number 1 is China, which doesn't have an open capital account.)
> How does Srilanka holding dollars benefit from a strong US consumer market?
America is Sri Lanka's top export partner [4].
[1] https://en.wikipedia.org/wiki/Fedwire#History
[2] https://en.wikipedia.org/wiki/Bretton_Woods_system
[3] https://en.wikipedia.org/wiki/Manufacturing_in_the_United_St...
[4] https://oec.world/en/profile/country/lka/
