This morning, when I was surfing the internet, I saw a phrase by chance. I thought it had a superfluous word.
They send their cars over by the millions, and what do we do?
The word "over" was superfluous, wasn't it?
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Your question title asks about the use of "by the millions". The body of your question asks about the word "over", and in the body, the two words "over by" are emphasized.
So, separating questions for clarity, "by the millions" means simply "in very great quantity", millions or more.
Then there is the phrasal verb "send over". It could be replaced by just "send", but it has its own flavor or shade of meaning. Here is a definition of the phrasal verb:
Merriam-Webster "send over"
"to dispatch across the sea, through the air, or from one place to another"
So, I would say it's not superfluous, but part of the writer's expressiveness.