@heartvalley Hi. The order confirmation email is sent when an order is created by the customer.
The responsible function to send order confirmation email is validateOrder() in file qloapps_root_dir/classes/PaymentModule.php.
The html and txt email templates for the same are:
In file PaymentModule.php, the code after line number 843 (may vary) creates data for the mail template and the mail is sent using Mail::Send().
You can modify PHP code and templates to suit your needs.