little explanation about Gjs and introspection

[Marcin Sporysz <msporysz06 gmail com>]

Hello,
I did some reading about new Gjs, introspection stuff over the weekend
and I would be grateful if someone clarify few things for me.
As I understood, new introspection mechanism generates XML files
(.gir) with classes, methods etc and then it is translated to various
languages like JavaScript (Gjs), am I correct so far? If so, lets
continue.
Let's say i have this line of code:

"const Clutter = imports.gi.Clutter;"

now, I understand it imports classes/methods for Clutter support - but
how can I know which libraries provided .gir files, which methods and
in which languages they are available? is it tracked anywhere in my
system, some repository perhaps?