Hi @radjr,
The need for payment is communicated to PayPal in PaypalPaymentForm.inc.php in the display function. (This uses the 3rd-party Omnipay library.)
PayPal then contacts OJS via PaypalPaymentPlugin::handle to communicate fulfillment of the payment, which converts the queued payment into a completed payment.
Regards,
Alec Smecher
Public Knowledge Project Team