I’m still trying to comprehend stereo vs mid/side. I’ve read that Ableton really does mid/side and not true stereo. But I’m a cave man and couldn’t identify this difference.
Mid-side can mean different things to different people. On the one hand it's a specific microphone configuration that requires a "mid" microphone (often, but not necessarily always, a cardioid pattern mic) set up coincident with a figure eight microphone (ideally, a ribbon). Through mid-side processing (which can be done manually by making the mid mono, then copying the figure eight track and inverting the polarity of the copy, then hard panning each of the figure eight tracks left and right) one can then in post adjust the stereo spread of the sound by adjusting the ratio of the mono mid to the hard panned stereo sides. Technically, adjusting the stereo spread changes the frequency response of the resulting virtual mic array.
Mid-side processing can also be applied to coincident stereo arrays like XY. So even though the mics weren't set up in a mid-side configuration, one can still (in a way) apply mid-side processing. This is another meaning of mid-side to some. (I haven't done this myself with XY recordings but I know some people who do.) Likewise, let's say when you record using a mid-side configuration you use your recorder's built-in mid-side processing to record your raw track with the mid-side ratio already set (something I never do, but some people must since it's a fairly common option). You're not necessarily married to that resulting stereo track because you can still apply mid-side processing to it like you would a recording from an XY array.
Of course any stereo cues that come from a coincident microphone configuration are based on volume differences, whereas spaced mic configurations get stereo cues from time and volume differences. Copying a mono track and applying some very small amount of delay to the copy, then hard panning each left and right, is one way of trying to make a mono source sound like stereo. A very simple and not always great way, but sometimes it works.
Indulge me in a brief rant... True mid-side and XY (and Blumlein...) microphone configurations require the mic capsules to be coincident, which typically means right on top of each other for practical purposes. Despite what this guy says in the video at 5:07, that configuration is not XY and will result in some degree of comb filtering!
Edit: To get back to the original question, mid-side and stereo aren't really different because it's kind of apples and oranges. Mid-side is a microphone configuration, or a way of processing an existing stereo track, to be able to adjust the stereo spread (mixing the ratio of the mono center to the sides) in post.
