I have to confess, I'm a little unclear about what exactly mastering IS in the modern world of DAW music-making.

I mean, if I have a Logic project with 4 audio tracks and 4 MIDI tracks, feeding into a stereo output track, I will be making tweaks to both the 8 part tracks and the output track right up until I bounce it. So where is the line between mastering and whatever precedes mastering in post-production?

The sense I'm getting (BTW in case it isn't obvious I'm an non-pro as is possible to be) is that the mastering being discussed here comes after a final stereo mix, and is applied to a stereo recording. Certainly that's what the Soundcloud thing is. This of course involves the limitation that all the parts are married at that point, and can no longer be individually adjusted (fancy AI doesn't count). Does mastering as such involve only global changes that affect the entire recording?

I do totally get it if we're talking about a process where you get multiple masters for a variety of services and hardware platforms. (Here's song123-for-youtube.wav, here's song123-for-apple-music.m4a, etc.) But Soundcloud doesn't offer that. You just get one recording, with very little in the way of controls ("Blue Sky" vs. "Thunder" etc.) and a specific emphasis on avoiding loudness penalties by streaming services.