INITEX is a slightly modified version of
tex suitable for creating format definitions; as such, it starts with a very blank slate. Its starting state is described in the TeXbook, p.39:
There's a program called
INITEX that is used to install TeX, starting from scratch;
INITEX is like TeX except that it can do even more things....
INITEX needs extra space to carry out such tasks [hyphenation tables, formats], so it generally has less memory available for typesetting....
INITEX begins, it knows nothing but TeX's primitives. All 256 characters are initially of category 12, except that
<return> has category 5,
<space> has category 10,
<null> has category 9,
<delete> has category 15, the 52 letters
a...z have category 11,
\ have the respective categories 14 and 0.
So the answer to your question is that these definitions are made in the TeX source code; I do not have
TeX: the program at hand right now, so I can't give you a line number.
These days, there
is may be no actual program called
initex on your computer; instead, as the TeX FAQ explains, it is incorporated into
tex itself via command-line options.