How to call one file functions in other files in custom module?
It's PHP, so you use require or include to include functions from other files.

Drupal's wrapper around those is module_load_include. Examples from the docs:

// Load node.admin.inc from the node module.
module_load_include('inc', 'node', 'node.admin');
// Load content_types.inc from the node module.
module_load_include('inc', 'node', 'content_types');


Only use this function inside another function, not in global scope. If you need to unconditionally depend on a separate file for a whole module, just use require_once or whatever makes most sense for your use case.