The business model used by publishers in the print publishing era.
Most often, this submission is made through a literary agent. The agent represents the author, but also filters out unsuitable manuscripts to avoid wasting the publisher's time. Some publishers won't accept a submission unless it is sent through an agent. Publishers that do accept unrepresented submissions typically keep them in a slush pile.
If a publisher decides the manuscript is marketable, they will offer the author a contract for the rights to publish a book.
The author is then paid an advance and completes the book, either writing it based on the book proposal, or working with an editor to polish the existing manuscript. The publisher controls the marketing of the book, including the book cover, title, pricing, and promotions. The author typically assists in the promotion process by attending signings, readings, and interviews, which may be arranged into a book tour.