Here's a few more details from Variety that I don't believe the other articles had.
"A passion project, started on the fly in anticipation of the Mouse House’s centenary this year, the nine-minute short brings more than four hundred characters back to life, throwing hand-drawn, CG and live action characters into a dizzy mix as they collectively buzz around the Roy E. Disney animation building on the Disney studio lot, causing gentle comic mayhem and leading towards a sentimental close set to the studio’s most emblematic song."
"Soon enough, the walls very much do, as Mickey and Minnie spring loose from hanged photos, rousing Tiana from “The Princess and the Frog,” Peter Pan and Wendy too, Moana, Maui, and Merlin, alongside Elsa and Goofy and Winnie the Pooh. Oh and about four hundred more – with “Zootopia” fan favorite Flash Slothmore drawing the screening’s two biggest laughs."
In response to tragedy, Annecy opens with heartfelt appeals and the world premiere of Disney 100-year short 'Once Upon A Studio.'
variety.com