There is an whole industry based entirely on preying upon financially vulnerable individuals.
是什麼意思

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"based entirely on...." = 'relies on/comes from....'
To sustain or uphold the whole industry, they have to rely on prey on financially vulnerable individuals to keep earning money, is what "based entirely" means.
Another example: "The book she wrote is based entirely on the royal family's lives, so you can find out the truth of how they actually live."

"preying upon..." to prey means to feed. If a lion preys upon a gazelle, then it is feeding on it.
The whole industry preys upon or feeds onto/targets financially vulnerable individuals.