the whole OS
c,c++, perl,php,python,ruby,assembly, html,there are many more .....
How can I see its source code
you read it in a text editor
I mean which directory do I have to search to see its source code
for the ENTIRE operating system ?????
for any one installed program get the source for it.
or from the program website
but you might want to start with reading the fedora documentation FIRST