This would be about trade. Your country can make whatever currency it likes, but when you go to buy goods overseas, you tend to have to pay in USD. Not always, but it’s the default.
BRICS don’t like this, for obvious reasons, but they lack alternatives. They can try shifting to the Chinese Yuan, but that only moves the problem for everyone who isn’t China. They can try to make their own currency union, but they aren’t even as politically united as the EU, and the Euro has issues keeping everyone moving the same direction.