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I know that plain TeX is the low-level typographical language that describes nearly every aspect of presentation, taking care of things like paragraph breaks, new pages, and the like.
I know (well, I've inferred) that LaTeX is in fact written in TeX, partly to demonstrate its power and partly for sheer convenience; LaTeX allows the author to construct documents logically rather than 'typographically' as using plain TeX, no-frills, would have you do.
What I don't know is how LaTeX really does it. Does LaTeX compile the document into a PDF, or does it compile the document into plain TeX for
tex to actually process? What features does LaTeX actually add on to plain TeX?