Magento 2: When Paypal Express will generate Payment Id?

Question

I'm processing payment through Paypal Express Checkout.

Order is already processed in Backend & will store Transaction Id in database. But i need to process some Orders using API.

To do that i'm using https://github.com/paypal/PayPal-PHP-SDK.

It will generate Payment Id using https://github.com/paypal/PayPal-PHP-SDK/blob/master/sample/payments/CreatePaymentUsingPayPal.php

https://stackoverflow.com/questions/39785327/magento-2-how-to-change-payment-action-method-on-the-fly-in-paypal-express

It keep asking me for Payment Id which start with PAY-XXXXXXXX. So does Magento store Payment Id?