I've looked at syncthing, however it doesn't have an iOS app. Regarding nextcloud - it only seems to do full syncronisation like dropbox does. How can I store

I've looked at syncthing, however it doesn't have an iOS app. Regarding nextcloud - it only seems to do full syncronisation like dropbox does. How can I store large shared directories on the server side only? eg, my FreeNAS box has ~20TB in it, and I'd like to be able to access it as a shared directory, but obviously not sync it to my laptop.