YouTube Music is finally getting automatic cast sessions similar to Spotify Connect. When music is playing on a smart speaker, you can now control playback by opening the YouTube Music app on your phone. This is a server-side change that will be implemented gradually.
If you want to be able to use your smart speaker and phone independently, you must be connected to the same Wi-Fi network and use the same YouTube Music account on both. The speaker’s playlist will override your phone’s queue, forcing you to take control of the phone.