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They do something that gets results. It doesn't matter if it's ML, AI, DL, linear regression, or just fingerprint+cosine similarity. There's some talent for getting some complex shit done.

They do audio fingerprinting[1], so no ML/AI in the core feature.

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[1]: https://www.ee.columbia.edu/~dpwe/papers/Wang03-shazam.pdf

"Music information retrieval" is what researchers call the overarching field. You might say the Shazam algorithm is akin to those fields in the sense that it relies on feature extraction. It's possible they may train a classifier to do the matching, but yeah, probably not purely ML/AI/DL.