I have spoken to the tech support from ESRI on this and they informed me that for a WAB Dev, the parameter to use will be the "?config=", where you had need to create new main config_name.json that will be referencing a new web map also or LocalLayer_config. See discussion here https://community.esri.com/thread/181279
It is a good workaround though it requires you to create webmaps and associated config files for each use case (the number of operational layers you need to show in individual URL )
It would have been great if the URL to services (operational layers) works directly like in the portal viewer.
Datasources are layer that are added to the application but NOT the map. They are available to widgets that use datasources like the infographic widget.
If anyone is still interested in turning layers on and off using url parameters and you're using the WAB developer edition, I've modified the url parameter handler module so you can toggle layers and sub layers on and off. I needed this functionality in an app I built and couldn't find a solution anywhere else. I posted about how to use it at https://ryandavison.net/toggle-layer-visibility-using-url-parameters-in-web-app-builder-developer-ed... . There's a link at the bottom of the post to the updated module on Github.