Tap Ultra in the device menu of the Google home app and you will see a screen like this: Tap "Cast audio" to connect your phone to Ultra.I've did other searches and it seems it's not possible at this time to Cast with an iPhone just the audio from YouTube(not YT Music) to the Wiim Ultra. YT does not show up on the Home app.
(Sorry, not English)
Then simply play the youtube video. No need to cast with Youtube app. This way you can play audio from any app.
I think it can be done on iOS as well.
However, there should be less delay with Bluetooth than Chromecast.
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