1) why are oil, gold and wheat almost priced in USD?
If you talk about the US Dollar,long before the dollar developed as it is today, after World War II in 1944, if I'm not mistaken, the US dollar was set as the global financial standard, to stabilize the economy in trade.
It is not surprising if you see that currently the US dollar is half used to carry out transactions as a feature of international trade, this is not without reason, international market trust which makes the world believe in the dollar, whether in oil transactions, gold and so on which are believed to be able to store value in using US dollars.
2) Why most of the loans are taken in USD despite local currencies?
Many countries have taken the initiative to use US dollars as loans, because US dollars can be used as a rare form of economic stability, As far as I know, the US dollar has 88% strength and can dominate the world's daily foreign exchange transaction sector, that is the reason why the US dollar is strong in considering loans for many countries.