Earlier quoted context omitted.
What 600-page documentation are you referring to? The TeXbook by Knuth goes only up to page 483 (with the appendixes starting on page 304). By the way, a really excellent book for (plain) TeX is A Beginner's Book of TeX by Seroul and Levy: it's truly for the beginner, yet at the end you'll be practically an expert, or at least know how to become one. I've seen people get frustrated with LaTeX (not TeX), but that's be…
Your comment is interesting to me. I've read about TeX and LaTeX from time to time and it was my impression that everyone used LaTeX because TeX itself was difficult to use. And I never questioned this because almost every article I could find only spoke about LaTeX. Your comment is making me reevaluate what I thought I understood.
If you're writing a paper for publication in a journal, they probably have their own typesetting style. With LaTeX you can just specify your paper as a sequence of paragraphs, "sections", a title, an author, and so on, and, by including the journal's style file (assuming it's good), your paper will come out typeset in that journal's style. This philosophy is explained at https://www.latex-project.org/about/
This is great for certain (many) kinds of documents. The problem with this, though, is that the moment you want to change the appearance and get into typesetting yourself, you're fighting a system that was actively designed to prevent you from doing this.
An analogy may be a system like, say, WordPress: if what you have is a blog, then it would work for you, and you even get to choose one of several “themes” that others have written, and so on. The set of available plugins and customizations can even be impressively versatile (for example, I noticed that time.com runs on WordPress, and apparently so do a number of other non-blog sites: https://vip.wordpress.com/clients/). But ultimately, if you're trying to figure it out by yourself and have your own ideas about what your website should look like, a system like this will always be more frustrating than the alternative (of having full control).
TeX is not difficult to use, though in the beginning you'd have to specify manually things like the typesetting of the document title (put it in a bold font at 16pt, centered on a line...). LaTeX has features to automate many such common typesettings tasks, but it has its own flaws (some stemming from the fact that it's implemented using TeX macros, which weren't designed for such heavy “programming”).