There are multiple places where you can look. As at IDE 1.6.4 (Ubuntu version) some are in:
But these are the high-level libraries, in particular:
That does not cover the low-level libraries like Tone, Print, main, etc. These are in:
There you would find the Arduino core libraries like:
However even that does not locate things like strcpy. The header files for them can be found at:
There you will find:
Even that is not the full story. In a subdirectory (avr) you will find the avr-related things, with files for different processors, such as (in part):
As for the source of strcpy, I don't think it is in the distribution. They include the standard libraries, pre-compiled. For example, if you look in:
You will see various pre-compiled libraries, for example:
To find the source you would need to find where the AVR source of libc is (possibly at http://www.nongnu.org/avr-libc/).
As for the Mac, all this stuff is a few levels down in the "Application Package" which you can view by right-clicking and selecting "Show Package Contents". Look in the
None of these places are where you should put your own libraries (ones you write or download). They should go into the
libraries folder which is a subdirectory of your
sketchbook folder. Create the
libraries folder if it does not exist. That way user-installed libraries persist from one version of the IDE to the next.