How is TeX/LaTeX parsed?

Question

I have some questions about TeX/LaTeX parser and parsing TeX/LaTeX after reading the following:

Limitations when parsing TeX as a context-free grammar

This is a PEG parser, which means it interprets LaTeX as a context-free language. However, TeX (and therefore LaTeX) is Turing complete, so TeX can only really be parsed by a complete Turing machine. It is not possible to parse the full TeX language with a static parser. See here for some interesting examples.

It is even undecidable whether a TeX program has a parse tree. There has been done some research on the problem of parsing TeX, see here.

Source: https://github.com/michael-brade/LaTeX.js/blob/master/README.md

What are the parsing rules used in TeX/LaTeX?
If this is not a context-free grammar what is the type of grammar used?
What is the parser used in TeX/LaTeX and how does it prase a TeX/LaTeX document?
How is the parser written/generated?

I take it you've seen https://tex.stackexchange.com/questions/4201/is-there-a-bnf-grammar-of-the-tex-language?noredirect=1&lq=1 — Joseph Wright, Feb 21 '20 at 07:41
there is no grammar it is a custom parser written in pascal, look at http://mirrors.ctan.org/macros/plain/contrib/xii/xii.tex this is plain tex but not amenable to any parser generated by standard parser generators from a grammar — David Carlisle, Feb 21 '20 at 08:02
Check the Blog entires by Graham Douglas, here: https://www.overleaf.com/learn/latex/A_six-part_series:_How_do_TeX_macros_actually_work%3F — Uwe Ziegenhagen, Feb 21 '20 at 10:23

How is TeX/LaTeX parsed?

Limitations when parsing TeX as a context-free grammar

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