Google has rolled out two major changes for Wear OS, months before the Pixel Watch's launch. Now, it is possible to stream music over LTE or Wi-Fi via YouTube Music, a feature that has been missing since the service's introduction last year on Wear OS. Also, the next-generation version of Google Assistant has finally landed, but only for the Galaxy Watch4 and Galaxy Watch4 Classic for the moment.
https://www.notebookcheck.net/Google-rolls-out-new-YouTube-Music-features-to-Wear-OS-smartwatches-as-next-generation-Google-Assistant-lands-on-Samsung-Galaxy-Watch4-and-Galaxy-Watch4-Classic.621726.0.html
Why not just use Spotify on your watch as you can even download a playlist into the watch. No WiFi or LTE required for playback.
Because Google owns YouTube unsurprising they are making it compatible with their new wear OS