Hey,
as i currently don’t have any app updates outstanding i hope it is enough to say that the notification for app updates are shown on the upper right corner of my ownCloud GUI. If a notification is available i’m getting a small bell there with a link over which i can update the app in question directly from within the GUI.
Based on another comment in Shared Hosting - Owncloud Updater - 10.13.3 not showing - #5 by TJ.Hooker74 could it be possible that you don’t have the Market App installed / enabled at all? I think this could be required for getting notifications and updates directly in the GUI.