@GOATFATHER : It's just a Chromecast Ultra, so you can do all the stuff that you can do with any Chromecast, it just happens that you can also launch Stadia through it when it is setup accordingly. So, for our CCU we've cast movies to it from our phones as one example, but in reality we don't do too much outside of using Stadia with our CCU because we've already got access to Netflix/Prime and a bunch of other stuff through our TV service provider and we just watch whatever we want through that via a TIVO box or it's rebranded equivalent with Virgin Media.
But back to your original question, you don't need to do anything special just cast whatever you want to watch on your TV via your CCU from your phone/tablet/computer etc.