Re: Streaming Audio ATMOS, 5.1 or 2.0 - The heart of the matter
1) Mostly EAC3 (DD+), AC3 (DD), and AAC (different variations thereof), and occasionally some others (DTS, etc)
2) No to muxing, mostly yes to HDMI/EDID detection/configuration determining which audio stream to pull.
3) They arent streaming any of those as "files", though AC3 is streamed as a format for some services.
4) Depends - Mostly the 2.0 is AAC (variants thereof) converted/transcoded to DD2.0 or PCM 2.0, sometimes its DD+/DD 5.1 downmixed to 2.0.
5) Depending on what you accept as "direct knowledge of the workings..." you can accept the above or not
6) The streaming services are not monolithic or sedentary, and the technical specs can/do change. Additionally, specific configurations of different decoder versions/options can result in different audio output, within a specific device, or between differing models/platforms.