The singers simply sing the notes on the scale of the keyboard, nothing new. The difference is, the note is linked to a video of them singing each respective note.
To make it interesting, they could sing each note slightly differently, vary the position of their lips for instance, more or less vibrato for some notes, even comical or character interpretations of the notes for others. Sing one note with a pirate face and an "Arrrrgh!" tone for example while another one sounding like a bird and making little flapping wings with the hands. Basso guy might hit a cliche body builder contest pose or something. No rules, they'd just get to have fun.
Then the person playing back the audio/video samples of the various singers would be playing a virtual choir acting out each note.-- doctorremulac3, Sep 11 2020random, halfbakery