How to locate the file who handles submit button in Drupal 7 - drupal

Question

I'm new to Drupal 7, it's weird to me compared to traditional programming like native PHP, MVC frameworks, Node.js, .Net, etc. Please help me locate the file and/or function that handles the submit button. I have this tpl file already
<form id="com_id" method="post" action="">
...
<button type="submit" id="saveBtn" class="hidden btn-save" name="btnSave"><?php echo t('SAVE'); ?></button>
As you can see there's no value in form's action. In Laravel MVC, it's easy,
Route::get('/the_path', 'RoutinesController#theMethod');

Answer 1

1.Create a custom module
2.Create a form in it example:
function mymodule_example_form($form, &$form_state) {
//build your form using the form api documentation, linked example and your needs
}
3.Add validation for the form:
function mymodule_example_form_validate($form, &$form_state) {
//your validation goes here
}
4.Add submit function to it
function mymodule_example_form_submit($form, &$form_state) {
//do stuff on submit here
}
Not sure where you want to print it but you can always put it in a custom block and render it from there or attach it to a node using hooks (mymodule_node_view) there are many ways but if you need more details be specific and we will help you out and don't forget to turn the module on once you created it :)
UPDATE: If the form already exists use hook form alter:
function your_module_form_alter(&$form, &$form_state, $form_id) {
if ($form_id == 'your_form_ID') {
$form['#submit'][] = 'your_module_custom_form_submit';
}
function your_module_custom_form_submit(&$form, &$form_state, $form_id) {
//do stuff here on submit
}