I'm wondering: Where are the header files - such as stdio.h or cs50.h - located: the ones that we include at the start of almost every source file?
First of all, these are not the libraries! These are known as libraries' header files (aka declaration files). You may have a look at this answer to learn a little bit more about them!
The header files for the C standard library are located in the default include directory. You may reach this directory by opening up the terminal and executing the following commands
cd /usr/include ls
There you can see the header files for the C standard library. If you want to view one of these header files (e.g.,
stdio.h) on gedit, you may execute this command while you're in
The cs50 library's header file (i.e.,
cs50.h) that contains the resources of the cs50 library that are available for you to use exists in the same directory (i.e.,
/usr/include). You may view it on gedit by executing the following command while you're in the default include directory