Apologies for the misunderstanding! I went back and double checked my setup and it looks like the Nextcloud iOS app doesn’t support opening a folder from the native files app.
Personally I use logseq as mostly a desktop app and the only logseq file I open on my phone is an org-mode file that I use for task management. If you use an org-mode task management app like beOrg, you can sync your tasks using Nextcloud+webDAV as the backend.
At the moment, if you want cloud sync to work, you will have to save your logseq graph directly to iCloud Drive and use the iOS file picker to open the folder. If you click on the browse tab at the bottom of the file picker, then click on the back button, the app will ask you if you want to use a location on your iPhone or on a cloud provider. From here you can select a folder in iCloud drive.
If you are still interested in Nextcloud support, not being able to select a folder is a known issue and heres the Github issue discussing it:
An alternative would be using Nextcloud’s built in webDAV server for syncing but that’s a little more temperamental. I was able to connect to my Nextcloud instance from the files app but was unable to get any of the files to actually load in the logseq file picker.
You can find more information about it here:
Hope this cleared up any misunderstandings!
Cheers!