Who's holding the trillions of yuan that they sold their dollars for?

Technically, China holds the most yuan and the US holds the most dollars. Let me put it this way:

When the US wants to buy a product from China, it buys Chinese yuan and sells dollars to China. China sells products to the US and buys dollars in return. China is selling the US dollars it has earned from the USA to the USA as US bonds in return for interest.

At the end of this equation, China has plenty of Yuan and US bonds. The US has the goods used in trade and plenty of US dollars.

The value of the Dollar against the Yuan, then there's the ever declining usage of dollars for international trade:

That's true, but it's not the kind of thing that's going to shake the U.S. dollar's throne. The use of the yuan as a reserve currency in trade has moved from 0.5 per cent to only 2.57 per cent. Some sources put it as high as 4 per cent, but the result is the same. The Chinese yuan has not yet surpassed even the Japanese yen and the British pound. Even the Canadian dollar, a much smaller economy than China's, has as much market share as the Chinese yuan.