Earlier quoted context omitted.
I did a master's thesis using Word, and my thesis advisor got on to me about not taking advantage of all the Word features like styling and proper page-breaking, etc. A few years later I published a book using Word and my publisher demanded that I not use any of the Word "gimmicks" like styling and page-breaking.
Now I'm curious about what software is used in actual ink-on-dead-trees book publishing.
The vast majority of print-interior typesetting is being done with Adobe InDesign.
Some publishers (still) have their own system that does not use Word styles - usually using ASCII tags inserted into the content. These are often legacy approaches dating from the 1980s, but if it works why change it?
A few have begun to use Markdown-based systems, but this is very rare in commercial publishing.
The vast majority of author-to-editor-preproduction workflows are done in Microsoft Word. Some publishers (including the one I worked for, and the ones I work with now) use Word styles for all content formatting. Part of the editorial "pre-production" task is to take what authors give us (font formatting with bold / italic, paragraph formatting with returns and tabs) and convert it to styles.