Question: Where does:

admin --> `index.php?route=api/order/add&token=xxx` point to? 

Which controller file, method will this call?

Answer: 
index.php?route=api/order/add&token=xxx points to catalog\controller\api
It will point to

1. Controller Folder (admin/api)
2. Controller File (admin/api/order.php)
3. Controller Function (admin/api/order.php => function token())