The short version: it doesn't have one.
Continuing a track record of sensible purchases of music gear, I picked up a used Otamatone Neo for cheap. One of the things that sold me on it was the ability to quantize pitches and snap notes to a scale, which may be cheating, but also makes it a bit more workable as an instrument.
Unusually, setting up pitch quantization requires the Otamatone smartphone app1 and a specifically TRRS2 3.5mm cable for "OTM link". My first assumption was that the smartphone app just encoded serial commands to the instrument as audio, like the Volca Sample does, but I couldn't quite figure out why the microphone input was needed.
In hindsight, the reason is obvious: the pitch quantization is done in the app itself, by listening to the Otamatone's audio over the microphone input, performing pitch tracking and synthesis on the phone, and then piping the audio back to the Otamatone's speaker.
It might be a clever way to keep costs down but it does feel quite silly...
1which incidentally gates scales other than the chromatic and minor blues behind a $5 purchase, making the phrase "music theory DLC" rattle around in my head
2abbreviation for the long German word for something so common you would expect most brick-and-mortar electronics stores to carry it, except they don't