In the Old World, my Big Publisher did all the work for me, the writer. (This remains a strategic option.) My only job was to mail in my printed Word document.
In the New World, I, the Self Publisher, have quite some work to do, and at each step, I can DIY (do it all myself) or pay someone else to do all or part of the job.
An example? An ebook needs a cover as much as a paperback does. I can DIY this task: conceive a nifty design, then take a photo or draw a picture on my iPad, then type in the title, etc., using some software program, and finally upload it to the Amazon website. At the other extreme, I can pay mega bucks to any one of a multitude of firms who can make my entire book, including using my title to design and manufacture the cover. In between, there are many, many firms or individuals who charge a lot or not much to do part of the process. As one example, there are “build your own cover” websites that give you a range of options but don’t cost much at all (but typically give shoddy results).
A wealth of options, expensive or cheap, wonderful or abysmal, exist for other self publishing steps: copy editing, proofreading, book layout, indexing, referencing, publicity, promotion, distribution (to bookshops, for a print-on-demand paperback), etc., etc.
What a minefield!