How to Use Nextcloud Notes with macOS Notes.app?
How to integrate notes in the Nextcloud Notes app into the Notes.app of macOS X? Notes in the Nextcloud Notes app are .md files.
Earlier Mac models
Having the word "Note" in an application name is no guarantee of interoperability with Apple's Notes application. The latter has very explicit details of what content you can import into it.
Import and export notes on Mac - Apple Support
