Javascript source code appears to be here:

https://om-intelligence.ch/projects/vocal-notation/vocal-not...

The comments mention it is using autocorrelation ACF2+ for pitch detection, and Krumhansl-Schmuckler for key detection. (Which doesn't mean very much to me yet, but is giving me some keywords to start researching.)

I was expecting YIN since that's the first thing Claude reaches for, and the code comments in the source do contain a malformed em-dash....