None. Once you connect any other Airplay 2 speaker the audio drops to stereo.
this post was submitted on 21 Nov 2023
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The HomePods achieve this because the "Default Audio Output" method (which require that both devices have to be in the same room within the home app) is not really AirPlay, but an ad-hoc dedicated network between them and the Apple TV created via AWDL (Apple Wireless Direct Link). That allows for real time audio to be transmited with no latency and surround sound support, including Atmos.
If you connect the Homepods trough the Apple TV control center they use Airplay and therefore have the same limitations that any Airplay speaker out there.