Since we also have partners with external content offers (specialist library) on our Neos-based platform, the requirement now arose that it must be possible to export this content into a file in the event of a possible dissolution of the partnership.
Specifically, all content should be exported into an XML or JSON file.
Now the question:
Is there a way to export all content (the entire site) into such a format so that this content can be used again on another platform?
Many thanks
Yves
neos has build in support to export the site to xml (flow site:export)
but this xml is of course only understood by neos itself.
so you likely need to build yourself an own adapter 
Either as Marc said the site export which would contain the nodes and referenced assets.
Or create a custom export with the required fields and data. There is no generic solution as this is so specific to the use case.
I created GitHub - Sebobo/Shel.Neos.Reports: Report generator module for Neos CMS node data for a customer a while ago which allows to configure node data that should be exported to Excel. If that is almost what you need, it shouldn’t be hard to extend.