I didn't find this elsewhere, please point me in the right direction if I just searched for the wrong things.
I just downloaded the Joplin iOS app and tried to setup sync with NextCloud. Unfortunately, rather than asking me for my NextCloud's domain and sending me through the oauth flow like other NextCloud integrations, the app wants me to store my NextCloud username and password in it. This is presents a problem as our NextCloud uses single sign on for account management, so using NextCloud's default login method will not work.
Is there any chance of getting a proper NextCloud auth flow in Joplin? It looks like a nifty app and I'd love to use it, but this makes it unusable for me.
Locating the webdav URL was not the issue. Although it would be nice if Joplin made it clear it was going to dump a bunch of files in whatever folder I passed it, and not make it's own folder like most sync-via-nextcloud apps do. I ended up with a bunch of crap in my nextcloud home folder because of this
I feel the documentation was extremely clear on the matter:
In the desktop application or mobile application, go to the Configuration screen and select Nextcloud as the synchronisation target. Then input the WebDAV URL (to get it, click on Settings in the bottom left corner of the page, in Nextcloud), this is normally https://example.com/nextcloud/remote.php/webdav/Joplin (make sure to create the "Joplin" directory in Nextcloud), and set the username and password. If it does not work, please see this explanation for more details.