We say “millions of dollars” in English because we need the plural form of “dollar”. “Millions of dollar” is incorrect.
I have millions of dollars in the bank
That house costs hundreds of millions of dollars.
In the first example, “millions” means some multiple of millions but we don’t know how much exactly. It could be 5,12, or 35 million but it is not one million or probably not more than one billion.
Million, Dollar – Singular and Plural
To form the plural of million and dollar you should add an “s” like a regular plural noun.
A dollar some dollars
A million some millions
Note that when you use a specific number with “million”, you do not use the “s”
Number with Dollar/Million
You use the regular plural with the dollar when you use specific numbers.
We do not use the plural “s” when we use the plural with million.
This rule for million is true regardless of whether we use million for money or in other contexts.
Hundreds of millions or hundreds of million
We use “hundreds of millions“ when we are being vague about the number.
I have hundreds of millions of books.
If you are using a specific number, you should drop the s in hundreds and millions.
I have exactly 5 hundred million books.