Is there any way to add the operational layer(s) using config file so that they can appear in layer list.
I am a little confused by your question. Maybe you can go into more detail and what you have tried so so. Are you just simply wanting to have an app that loads its operational layers based on a config.json and then have a layer list that displays those layers?
I like to add the operational layers in my app ( created using WAB developer eddition) to make it available to my all users. There was feature to add operational layers in config.xml file of flex viewer. Similar to that can we add to main config.json file ?
OK, so you are posting in the wrong GeoNet space then. There is a ArcGIS Web AppBuilder space specific for WAB questions (I have moved this question there). As far as adding a operational layer via the main config.json (like in the Flex Viewer), this is not an option. WAB does not allow for adding layer directly to the app from an ArcGIS Server url like the flex viewer did (without using a custom widget). If you want to add a layer at runtime then you use the OTB widget called "Add Data" if you want to add a layer to the app during configuration of the app then you add it to the AGOL/Portal webmap or use a custom widget like the Local Layer widget.
Release LocalLayer v2.2 · cmndrbensisko/LocalLayer · GitHub
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