關於現在式既問題想問問

It is correct to use 現在式 for facts, habits or opinion.

Your sentence: There are people from different countries in (the) USA.
= People from different countries are in the USA.
"people from different countries" is subject of sentence;
"are" is the verb (linking verb): and
"in the USA" is the subject complement.

This sentence doesn't make much sense and shouldn't be what you want to mean.

Correction:
1. People in the USA are from different countries.
subject = people in the USA
verb = are
subject complement = from different countries (obligatory adverbial)

2. People in the USA come from different countries.
subject = people in the USA
verb = come from (phrasal verb)
object = different countries (object of preposition, or object of phrasal verb)

其實整句係不對grammar, 中文翻譯,

如果你想講有好多住係美國的人係其他國家的人, you should say,

"There are many people in the USA who came from different countries.''

''are''用現在式無問題.