Documentation

• https://docs.nextcloud.com/server/stable/admin_manual/maintenance/index.html

App

• Backup App: a backup app that enables admins to create backups of a Nextcloud instance and also restore it
  https://github.com/nextcloud/backup 

Additional resources:

Inofficial apps

• https://apps.nextcloud.com/apps/ownbackup 

Scripts

• https://github.com/DecaTec/Nextcloud-Backup-Restore 

Forum discussions

• https://help.nextcloud.com/t/scripts-for-backup-restore/21433 
• https://help.nextcloud.com/t/third-party-backups/3258/19