Earlier quoted context omitted.
Shairport-sync is great for iOS clients, but I know of no way to stream the audio from my Pixel to my Pi.
For Android, or any other Linux based situation, you can just use PulseAudio for this. https://www.youtube.com/watch?v=o5-phFVfZnQ
The source for the app used in the video is here: https://github.com/ford-prefect/pa-output-switcher
That repo links to a page about building an AOSP firmware image which uses pulseaudio: https://www.freedesktop.org/wiki/Software/PulseAudio/Ports/A...
I don't want to use a custom firmware image. I like the firmware provided by Google, and the monthly security updates. If I were to switch to a custom build of AOSP, I'd need to sideload Google Play Services (by downloading it from a source I trust less than I trust Google) and would also no longer get security updates.