How do I find out which module handle a GET/POST/PUT?
Menu items (in code)

Drupal module register endpoint with hook_menu(). You can search the endpoint within the project, such as abc. It should stay inside hook_menu() with type MENU_CALLBACK.

You may find something like below.

function mymodule_menu() {
   $items = array();
   $items['abc'] = array(
     'title' =&gt; 'Ajax endpoint',
     'page callback' =&gt; 'mymodule_ajax_callback',
     ...
     'type' =&gt; MENU_CALLBACK,
   ),
   return $items();
}


Menu items (in database)

In database, table menu_router saved all registered menu items. You can lookup the endpoint on path column and find the corresponding callback function on page_callback column.

Modules

There is a number of module which provide UI or APIs for developer creating endpoints. Such as services and RESTful Web Services.