The email templates used can be customised in the same way as all other email templates under Configuration → E-mail management → Email templates. You can find them under user emails. For more information, see the Shopware manual.
In the Shopware standard, the variable {config name=mail} is defined here, which refers to the e-mail address set in Configuration → Basic settings → Shop settings → Basic information in the field Shop owner email for the respective subshop. If you manually enter an e-mail address in the Sender field, it will not appear as the sender of the e-mail. In order for the desired e-mail address to appear, click on the globe symbol to go to the Translations window.
There you can select the desired subshop from the available languages on the left and enter the desired e-mail address in the Sender field.
Small text modules can be edited via Configuration → Snippets in the Snippet administration. To see all the relevant snippets, enter the therm viison_dunning_document in the search bar. For more information about the snippet administration, see this article.