A tip.
If you want to keep your sync folder out of sight on Nextcloud, put a full stop before your Joplin folder name when you create it on Nextcloud and use that as the sync target, such as:
https://example.com/nextcloud/remote.php/webdav/.Joplin
In “Settings” for the Nextcloud “Files” app you can set Nextcloud not to display hidden folders, hiding your sync folder. I did that as I use E2EE and so there is no reason to access the Joplin sync folder directly on the Nextcloud box (not that it’s a good idea even if the notes aren’t encrypted). It also makes the sync folder harder to accidentally delete or modify if it’s hidden!!