Hey @R_Riedler,
just to clarify: The Sitejet Builder for Plesk doesn’t have the option to download a backup of the website within the Sitejet Builder. However, you can download the files of the website that are on the Plesk server: https://support.plesk.com/hc/en-us/articles/12377716258711-How-to-download-a-folder-using-Plesk-File-Manager - if you have any other questions on how to download files or folders from Plesk, please reach out to the Plesk support as our support options are limited at this point.
Hope that helps!