YouTube Music may soon get integration with HomePod and HomePod mini through Siri, according to developer Aaronp613. There is a section in the YouTube Music app that suggests this integration could be coming in the future.
Apple has already supported several third-party music services on HomePod, including Pandora, iHeartRadio, Deezer, and TuneIn Radio. However, Spotify and other music services have not yet taken advantage of this opportunity, even though Apple made the SiriKit Media feature available in iOS 14.