What is the difference between the US and the USA? Some people say they are the same. But why do people often say "US citizens" instead of "USA citizens"? or they often say "the president of the US" instead of "the president of the USA"?

It's like Federal Republic of Germany versus Germany, if you understand what I mean.

You see tags labelled: Made in USA but POTUS or SCOTUS too.

There is no difference between these two words. You can use both of them on your content.

Students: Are you brave enough to let our tutors analyse your pronunciation?

There's no difference in these two abbreviations, actually. Both of them refer to the United States of America. If the US is a shorter version of the United States, the USA is a full abbreviation of the given state. that's it. The difference is only in one letter but they mean the same thing. Both of them are used in people's everyday life. The "US" version may be less official than the "USA", however, but these are only my assumptions.