[Payment apps] How do we relate the payment with the checkout session or the order

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I'm building a Payment app (Payment gateway) and according to the documentation when initiating a payment flow a payload is sent to your endpoints ( link to payload https://shopify.dev/apps/payments/processing-a-payment), but looking at the fields and the description for the fields I don't see any relation to a checkout session or the order so I can fetch what is being paid for (i.e items). Is there a way I can fetch the items the user is buying when initiating the payment?