It’s not 100% US dollar system but is dollar based. Many transactions will be in US dollars even if the countries involved are outside the USA - it’s just convenience.
The euro and U.S. dollar together made up more than seven out of 10 SWIFT payments worldwide in January 2023, outperforming many other currencies
I read a couple of years ago that something like 90% of China’s overseas transactions were done in US dollars:
Despite that China ranks as one of the main economy’s in the world, the yean ranked as only the eight-used currency for international SWIFT payments.