Overview - https://tex.stackexchange.com/a/211392
You are linking there to plain.tex which is a file written in TeX not the source of tex-the-program (which is tex.web)
These days if you want to compile from source it is probably best to start with a full download of the texlive build sources.
The sources are at
http://www.tug.org/texlive/svn/