I just tested it and confirmed the streams do actually stream in Experience Builder. I too could find no documentation that explains how to use streaming layers in Portal. I found documentation that mentions that you must use the Classic Map Viewer to display and control "Streaming Feature Layers" E.G. Control stream feature layers-Portal for ArcGIS | Documentation for ArcGIS Enterprise<>. However, I could find nothing that describes how to make a Streaming Feature Layer in the first place. Luckily I have access to an expert who showed me this:
1. Get the REST URL to the Stream Service E.G.
https://your-host/arcgis/rest/services/your-layer/StreamServer2. In My Content click New Item -> Url
3. Paste the Stream Service URL and follow the prompts to save a new Streaming Feature Layer
4. Open the Classic Map Viewer and add the new Streaming Feature Layer, save the map
5. Add the saved map with the Streaming Feature Layer into an Experience Builder app
6. Expect the app to display the real-time streaming features on the map
Enjoy!