In a texmf.cnf file you can find lines such as
TEXINPUTS.pdflatex = .;$TEXMF/tex/{latex,generic,}//
or
TEXINPUTS.lualatex = .;$TEXMF/tex/{lualatex,latex,luatex,generic,}//
This means that, when a binary is called with the name pdflatex (the binary itself will be pdftex), the variable TEXINPUTS will be set to the stated value; a different value will be used if the binary is called with the name lualatex (and the binary will be luatex).
The binaries are set up so that, if you make a symbolic link to them, they'll use the format according to the name of the symbolic link and the variables as set in texmf.cnf according to the same name or the “non dotted one”.
For instance, there is only
BIBINPUTS = .;$TEXMF/bibtex/bib//
which will be used by all binaries linked to the kpathsea library (it is of course only relevant for bibtex, bibtex8 and bibtexu).
If you want to have a hugetex binary, you need to add memory settings such as
main_memory.hugetex = 20000000
and optionally set other variables such as TEXINPUTS. Then you make a symbolic link to the desired engine, say pdftex or tex, called hugetex and make a format (see the documentation of TeX Live for how to do this).
Where should these settings be added? To the top texmf.cnf, which you find the location of by issuing kpsewhich texmf.cnf. On my system this shows
/usr/local/texlive/2017/texmf.cnf
because this is the first texmf.cnf file read in and later named alike files will not override already set values.
Nowadays this is not very important, because one can add memory almost at will with different methods (see TeX capacity exceeded with glossary package for instance) without the need of a specially tailored engine. In the past, having hugetex could be a life saver for particular applications.
hugetex.exewas an alias fortex.exe --mem-max=20000000(see https://www.ndsu.edu/fileadmin/math/Home_Page/Resources/Tex_and_LaTeX/Tex_and_LaTeX_Bibliography_and_templates/miktex.pdf, probably other versions as well) – Marijn Jun 16 '17 at 19:32