> There’s no rule that they have to only contain declarations

Same thing in Python, you can write statements at the top level of a module, and it will be executed whenever someone first import that module...

> or that they have to appear at the top of source files

Same thing in Python, you can `import` anywhere, and the context/order matters and overwrites existing scope declarations.

> or that they have to be .h files

Same thing in Python, the extension doesn't matter if you use import lib, only if you use the `import` keyword does the name/path matters.

static const float FIR_FILTER={

#include "fir_coefs.csv"

};