I get how that works on desktop, but on mobile, I can add a local file as a vault in Obsidian, but I don't think that file could be tracked by my cloud sync app. Does the Nextcloud app support that? Not sure how you use git here, could you explain?

What I have gotten to work was to download a file from the sync app, open it in a markdown editor app and then save it to the cloud by sending it back to the sync app. It technically works but it was a bit too inconvenient to become a real habit (too many taps, need to rename the file on upload and set location each time,...).