Authors sign a contract which specifies royalty rights in return for copyright. The usual deal is that an advance is paid, and then royalties are paid until the book is remaindered. Then the copyright usually reverts to the author - who can't do much with it, because the book's time will have been and gone.

Some publishers hold on to copyright no matter what. Those are bad publishers, and worth avoiding.

You can also get work for hire deals, but they usually work out at about 25% to 50% of the total return on an advance+royalty deal. Authors won't usually sign them unless they're very stupid or very desperate for cash.