A traditional publisher hires editors who read your MS, which you send to them. They receive thousands of MSs each week. It may take up to 2 years before they get around to reading yours. After they read it, they either reject it or accept it. If they accept it, you well be sent a contact (and often with a recommendation that you go over the contract with your literary agent/lawyer before you sign it). Once you sign the contract and send it back, than the publisher's lawyer checks it to be certain that all is in order (and done legally). The publisher is given a "temporary copyright" allowing them to print and distribute your book to the public. In most cases, you still own the copyright, but while the book is in print from the publisher, you can't do anything with it until it goes out of print.
They hire an editor/typesetter to type set and spell check your MS. Cuts
and additions are not unusual at this point. You the author have no
control over this, and it is not unusual for an author to read the
finished book and ask: "When the hell did I write this?" or "Where is that chapter I wrote on....?"
Next they hire an artist to create the cover art. You the author has no
control over this either. Finally, they distribute the book to
bookstores worldwide. You never own them a cent. They pay you royalties.
Some also pay with advances. Most do 100% of the marketing and
promotion, as well. You the author are free to sit back and relax, while
the publisher does the work, leaving you with plenty of time to write
your next book. Something Self-published and Vanity Press authors can
only dream of, while they are rushing about writing press releases, and
hunting down places to sell their book.