latexdef which does not handle
--- as the operand. And
\texemdash is defined as a character. So that leaves me once again leaning on the collective wisdom of the group :)
There is no definition for
---. They are ligatures defined in the font information, just like the ligatures
fl that turn into single glyphs without the need to specify it.
It's traditional that fonts define these ligatures (in addition to the f-ligatures)
--for the endash –
---for the emdash —
?`for the inverted question mark ¿
!`for the inverted exclamation mark ¡
``for the opening double quotes “
''for the closing double quotes ”
T1 encoded fonts usually have also
,,for the double low quotation mark „
<<for the opening double guillemets «
>>for the closing double guillemets »
Other ones may be defined on a font basis, but shouldn't concern the user.
TeX transforms the combinations at a very deep stage of processing, basically when it's putting characters one after another to form words and paragraphs, very much later than the stage where macros are expanded.