For example, I understand in English I have to say 100 million persons. But the word "millions" also exist. In Spanish I would have to use the plural for million in the first sentence. Then when the plural for words for numbers like "millions" has to be used?
The words for counting numbers are usually singular in form because the plural marker goes at the end of the noun phrase:
two hundred bottles
ninety nine red balloons
one hundred and one dalmations
a hundred dollars
= one hundred dollars
three hundred dollars
four score and seven years ago
You often hear the plural words for these quantity numbers (tens, hundreds, thousands, and so on) in expressions of inexact quantities:
Hundreds of books.
This painting is worth millions.
Tens of flies were in the room.
There are hundreds of ways to do that.
Scores of children.
Five million people. [five million is considered an adjective].
Millions of people use the program every day. [millions is a noun.]
Both in Spanish are milliones with es, the plural form for both these cases.