it uses https://muscriptor.kyutai.org/

https://kyutai.org/blog/2026-07-10-muscriptor/ says We're releasing MuScriptor, the best open model for multi-instrument transcription to date, created in collaboration with Mirelo. Give it a recording: pop, classical, metal, jazz, whatever, and it transcribes the notes played by every instrument into MIDI, without needing to know in advance what instruments are present.