https://www.theverge.com/tech/2017/10/4/16424644/spotify-voice-commands-google-assistant-supported-devices

Google has announced that Spotify voice commands will be fully supported on Google Assistant within the coming weeks. This means you can use your voice to control Spotify on any Google Assistant-supported device.

Before, Spotify voice commands were only available on Google Home, but now the same functions translate to Google Assistant on your phone. Both free and premium Spotify users will be able to use the voice commands, like, “OK Google, play some rock music on Spotify.” If you link your Spotify and make it the default player on Google Assistant, you won’t have to say “on Spotify” at the end of commands.