Q: Do you have any documentation related to order and payment tables and schema?

I created some orders but did not find any entry in OrderPaymentGatewayTransaction table. So, if you have any document, I can look into that to clarify doubts like these.

A: We have schema for Orders module in github:
Database Model section.

To have entries in OrderPaymentGatewayTransaction you need to add payments to the orders. Some entities have collection properties like this:

To add them, you should add payments to the cart:

You need to install and configure some payment gateway module for payment creation.