Well... technically, if Trump destroys the dollar, it could end up making other countries buy more. It would be cheaper. Yes, people would be poorer, and inflation would skyrocket. But if the goal is to export more, this would work.
Inside traders. They can literally place bets against the dollar and make a ton of money. It's like betting on horse racing, but breaking the legs of the ones you don't want to win.
When it comes these theories about forex, I always realize I know to little about macro economics to understand why they would do x or y.
I thought Trump had lots of dollars, not sure why he would want to decrease their value like this... there must be an art of the deal move to be made in here too somewhere?