https://www.theverge.com/2017/10/13/16469156/google-assistant-chromecast-control-android-phones

The Google Assistant has always been able to control Chromecasts from a Google Home, but now that ability is expanding to the Google Assistant on Android phones, too. Android Police spotted the new feature, which seems to still be rolling out. It appears to work for traditional Chromecasts, Chromecast Audios, and devices with Chromecast built in. We were able to get it to play YouTube and turn on and off a TV.

It’s a really helpful expansion for a couple of reasons. For one, it means that there’s less fragmentation between the Google Assistant across various platforms, which should lead to less confusion for users; you ought to be able to ask it to do all the same things on all your devices. But also, it’ll be a handy feature for anyone who’s using Android and a Chromecast, but doesn’t own a Google Home.