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What javascript apis they use to create sites like that? To play instrument notes at different pitches, and to play notes simultaneously?

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].

Without looking too deeply into it, I assume it's MIDI and using some JS implementation of a MIDI engine / soundfont to play.