I am building a dashboard(web app) to present the landslide related data. I have some external maps (slope, drainage and etc). I want to show them as layers in the web app. I was unable to find a correct way of doing so. Can someone suggest a solution?
You mentioned in your comment that you're trying to map Shapefiles. If you want to load your Shapefiles into the map from the JavaScript API, there's a sample here that shows you how to load that file into the map.
If you'd like to add it to your Web Map so that you can view it in an app, you can do that too, just go to the Arcgis.com Map page and click Add > Layer from File (more info).