Abner Li schreibt in Source: YouTube Music planning more apps, including for Apple TV and Garmin für 9to5google.com:

Another target for Google is supporting how the HomePod can play music from non-Apple Music services, like Pandora, that have taken the time to build out the media integration. Those third-party services can be set as the default for voice commands. This comes as Apple at WWDC announced Siri support for AirPlay on HomePod, with YouTube Music shown as a partner.