The pitch of this singer is controlled using a technique I invented called Gesture Synthesis. I have made an interactive demo for it [0]:

Hit Begin. Singing should play.

(Think of pitch changes in the melody as a smooth line.)

Change the Tempo slider.

The pitch "line" smoothly expands and contracts with the tempo changes. Almost instantly.

This is Gesture Synthesis.

0: https://pbat.ch/recurse/demos/gesture_demo/