Difference between static function and normal function in C?

Difference between static function and normal function in C?

In our project, we have pretty big C file of around 50K lines, written in
90's. I wanted to split the file based on the functionality. But, all the
functions in this file are declared as static. So, file scoped. If I split
the file, then the function in file1 cannot call function in file2 and
vice-versa.
But, My TL feels like that there could be memory optimization by using
static functions. I wrote some sample code to see if the stacks are
different for different threads. It seemed like it was. Could someone
please enlighten me the difference between static function and a normal
one other an file scope?