Those other variables would have to be declared public (use extern, public is for C++), and you would have to include that .c file. However, I recommend creating appropriate .h files to define all of your variables.
For example, for hello.c, you would have a hello.h, and hello.h would store your variable definitions. Then another .c file, such as world.c would have this piece of code at the top:
#include "hello.h"
That will allow world.c to use variables that are defined in hello.h
It's slightly more complicated than that though. You may use < > to include library files found on your OS's path. As a beginner I would stick all of your files in the same folder and use the " " syntax.
