I'm not sure I'm understanding you here. Are you suggesting that Apple streams a different pre-rendered two channel version for each supported phone or computer? Or are you suggesting that they stream one two channel version that is then further processed for the target output device?
I'm not sure what's going on, but there may be some clues for us.
The Atmos content can be downloaded, as well as streamed. I don't know what's downloaded on an iOS device. On a Mac, thought, the download track is stored on disk as a package or bundle, basically a directory with special metadata so it is presented as a single file in the Finder. However, it's easy to open it up and see what's inside.
What one finds is a couple of XML files and four directories. Two of the directories appear to contain audio data, contained in .frag files. In each case, based on filenames, the data seems to be broken up into files holding 14.9769s of audio. In one directory each file is about 479kb in size, which works out to ~256kbps. The files in the other data directory are 1.4mb, or ~765kbps.
As to how they are used, I don't know.
I do know that, Mac, or iOS, the Atmos download has to provide for binaural (AirPods/headphones), "spatial" with internal speakers, and stereo (headphones, internal speakers, any other output). Apparently
multichannel output is not supported on a Mac (if it is, Apple doesn't explain how to use it).
I know that if you've downloaded the Atmos version of a track and then disable Atmos playback while keeping the download, Apple Music will play back a 256kbps AAC stereo version (no matter whether your playback settings specify Lossless). Could the stereo, binaural and "spatial" internal speaker versions be rendered from the same 256kbps file? Could a stereo version be rendered from a two-channel device-specific 256kbps Atmos file? What are the larger files for?
It seems to me that the simplest explanation is that the Atmos downloads contain 256kbps stereo version and a higher-bit-rate multichannel Atmos version that gets rendered for either binaural or device speakers. Without a download on disk, only one or the other are streamed from Apple to the device.
Or maybe not. My Mac is a 2016 MacBook Pro and Atmos playback on the internal speakers isn't supported. Perhaps if I had a supported model (MacBook Pro (2018 model or later), MacBook Air (2018 model or later), or iMac (2021 model)) Music would download a third set of audio data, one tailored for the device speakers. Does anyone with a supported system care to check? Chose the Apple Music preference to download Atmos. Find an Atmos track, download it. Find the download in your Music folder ("Info" on the track should show the download location). Right click on it and choose "Show Package Contents". There will be at least four directories, at least two of them will contain .frag files. Do you find a third with .frag files? Let us know.