I got interested in these over the holidays. Some of them are hoardes of singers and musicians, and others, select and practiced groups of dancers: some groups are professional, and others are troupes from various classes.
As usual, I'm late to the party. These have been going on for years, apparently. A lot about them fascinates me, but mostly, I like the "agreement" that it is supposed to look very spontaneous. Obviously, given the routines presented, they aren't spontaneous: a lot of planning goes into them. But for the most part, they do a great job of blending with the crowd, and merging into the performance gradually and cumulatively. When the thing is over, they blend back into the crowd. Therein lies the artform, in my view. The message seems to be that the ordinary may exist, but underneath, we all are carriers of vibrancy.
What a marvel, that so much goes into sharing the joy.
There are many of these on YT. I selected a few to share.
There is a dark side.... For however one might be enjoying them at whatever location they are presenting, there is a sense of "imposition". You can see that phenomenon more in the Christmas song flash mobs, but even in the dance mobs, there is that inundation which might be abrasive to a passer-by.