Where are the Libraries? Header Files?

I see some in my root
etosXX folder and others in something called cygwinXXX. Within cygwin, I see multiple copies of some header files.

Within “Project”->“Properties”->“C/C++ Build”->“Environment” tab->“Project” tab, I see both of these directories listed.

If I want to see what size size_t is and go looking for stddef.h…how do I know which header in cygwinXXX is being included? Is this buried in some make file or linker script that ESP is generating? Where?