This is determined by specific different words, some countries added before, and some did not. This is a fixed statement.

The unit is abbreviated form, which means the United States. Use it directly without an article in front of it. But if we write the complete form of the word, the complete form itself includes it.

Americans are Americans, not countries. He is talking about an American, but if he simply indicates his nationality, he will not add an article. He is an American. He is an American. )