Question
When the Approval is rejected and the Approval Comment is filled, where is that comment stored?
Answer
If you want to read the comment of a rejected Approval you need to read data of the object [Contract approval] where Contract is the object where you set up approvals. The field you want to read is "Notes".
In case you want to read the comment after somebody rejects the approval you can add "read data" element and send an email or create a notification using the "Notes" field.