When you know the TransactionId but need Order details, you can obtain it in 2 steps:

1. First, GET the payment CAPTURE info from /v2/payments/captures/{capture_id}.
   Although it's not shown in PayPal's example, the response body should contain the order_id within the JSON structure like
   "supplementary_data": {"related_ids": {"order_id": "0WV12345XF123456D"}}).
2. Now you can GET the ORDER info from /v2/checkout/orders/{order_id} using the order_id you obtained from step-1.