Looks like it’s using Tone.js[0] with a sample library called ‘Salamander Grand Piano V3’[1].

Tone.js is great. I’m using it to convert MIDI files and sync visuals on a jazz piano tutorial website I’m building[2].

[0] https://tonejs.github.io

[1] https://archive.org/details/SalamanderGrandPianoV3

[2] https://www.jazzkeys.fyi