This is the idea of Atmos. They deliver the source with an absurd number of channels. Your local player knows your room and speaker arrangement (from setup tuning ahead of time) and remaps the stupid number of channels into the right playback based on the equipment available. Got 20 speakers all around the room? Cool. Have a 5.1 setup? No problem. Just have the stereo speakers on your TV? Supposedly, no problem.

Whether they actually achieved that depends a lot on your equipment and its tuning.

Atmos doesn't have channels - it has played tracks with their 3d location encoded to the receiver can convert it into the correct channel setup.

It's fundamentally different from the "static" channel mixes we had before.

Yes, on Netflix you can choose. Choosing the stereo mix does offer a significant improvement on hearing dialogue. Regrettably, it has no idea of sensible default or even choice of default, so it always goes to 5.1 even if your setup doesn't have it.

Yes, but audio tracks used to take up valuable space on physical media, so you might indeed sometimes find distinct 5.1 and 2.0 tracks, but usually for the main language only. And of course this duplication is one of the first things pirate releases do away with, in order to reduce the final file size, so you won't find them on most mainstream scene releases either.