People is the plural of person that's most commonly used to refer to multiple humans. But people can also be used as a singular noun to refer to a population or particular community.
The type of figurative language that is used in the sentence from George Orwell's novel Keep the Aspidistra Flying is a metaphor. A metaphor is a figure of speech that compares two different things in an implied way. In this line, the public is compared to a swine and advertising is compared to the rattling of a stick inside a swill-bucket.
