Is the dollar still trusted worldwide?

There's no other currency in the world which nations prefer as a medium of exchange. The US is the only country with a bluewater navy, which is required for enforcing trade controls. The dollar is still trusted worldwide and more stable. The yen fell over 15% against the dollar in 2023, but the dollar's status as a world reserve currency helped mitigate the losses in other investments, such as oil and gas sales (Russia and Iran), and saved a lot on purchases (China and India, mostly).

Is there a currency that could replace the dollar?

Yeah, there is no currency that could possibly replace it. Yet people still want to act like the dollar is in danger somehow. Of course if another country arose as a dominant and stable power in the world it would challenge the dollar, that just makes sense.

Where do US dollars not get spent?

US dollars are not spent in major cities like Rio and São Paulo (and other major cities) because people generally pay a premium if they pay in dollars instead of the local currency. However, due to sanctions, it's very difficult for people in countries like Iran, Russia, and North Korea to hold US dollars.