[noja]

Nextcloud is a web app that lets you work with files and offers lots of other plugins too, like e-mail, calendaring, office, contacts. It's not a backup program. See https://nextcloud.com/ or https://en.wikipedia.org/wiki/Nextcloud

[NoGravitas]

You might, for example, use restic or borg to back up your Nextcloud's installation and storage.

[ausjke]

or, restic is your NAS backup while Nextcloud is your open source dropbox, roughly.