Using AirPlay. But as far as I understand from the media coverage, it does not support Home Sharing. So, you cannot say "Hey Siri, play me some King Gizzard and the Wizard Lizard" and expect it to be played from your computer.
Not having to go to your computer and enabling AirPlay and selecting an album is kind of the point of a home assistant ;).
"HE-AAC (V1), AAC (16 to 320 Kbps), protected AAC (from iTunes Store), MP3 (16 to 320 Kbps), MP3 VBR, Apple Lossless, AIFF, WAV, and FLAC"