1- there are around 200 countries in the world, almost 190 countries determine the value of their own currency in terms of the USD i.e. the currency is expressed in terms of USD as it is used as a common denominator. why is this the case? why is the USD used as the reference.

2- there are very few currencies that determine the value of USD in terms of their own currency i.e. their own currency is used as a common denominator. what are those currencies and why?

what is the logic for this? why is there differences between currencies in this respect?

3- what is the 4 strongest currencies in the world? what is the main reason in your opinion?

4- what are the 4 weakest currencies in the world? what is the main reason in your opinion?