AirPlay is not a single, uniform protocol. Its behavior depends on:
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the source device (iOS vs macOS),
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the application,
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and whether the stream is real-time or buffered.
On iOS, Spotify typically uses real-time AirPlay, which can preserve higher audio fidelity depending on the receiver.
On macOS, behavior can differ, and buffering, transcoding, or timing changes may occur depending on the app and OS version.
Because these behaviors are controlled by Apple and app developers, they can change between OS releases, which makes AirPlay powerful but sometimes unpredictable in mixed setups.