It is possible to integrate NextCloud Files into SharePoint Server so that the user is able to upload files through SharePoint directly in NextCloud Files? Is there any official integration app available?
Yes, the is a SharePoint backend, unfortunately users (including me) are struggling to get it working.
SharePoint Backend - Apps - App Store - Nextcloud
Issues see
Setting up External Storage for Sharepoint Online - 
Support - Nextcloud community
Nextcloud Integration with SharePoint - Support - Nextcloud community
closed in favour for Nextcloud Integration with SharePoint
(though still unsolved as on April, 22nd 2024)