What is the typical process for publishing a book? How long does it take to write a book on average? Do authors often make revisions to their books after they have been published?

There are many paths to publishing, far more than could be explained in one answer on this platform.

The amount of time it takes to write a book also varies greatly, and depends on the amount of research necessary, the author’s experience and skill, and the length of the finished product.

Every author makes a ton of revisions to their manuscript during the editing process, but that takes place before the book is published. Once published, changing things may mean issuing a new, updated edition of the book.

The one thing you don’t mention in here is marketing for your book. If you haven’t researched and written a marketing plan for your book before finalizing it for publishing, you are doing yourself and your book a disservice. There are many things you need to know before you finish writing your book that you will discover when researching how you’re going to market it. Having a written plan also forces you to solidify some nebulous concepts like what you want the book to do for you and who is going to read it.

I would suggest that any author interested in writing a book attend some writers’ conferences near them to learn some of the basics about the book industry. Talk to experts there, learn from the workshops, be exposed to other people in the publishing industry. That will get you started on the right path to learning the things you know if you want to become a professional author.