https://phys.org/news/2019-03-artificial-intelligence-google-doodle-features.html

Google says the Doodle uses machine learning to "harmonize the custom melody into Bach's signature music style." Bach's chorales were known for having four voices carrying their own melodic line.

To develop the AI Doodle, Google teams created a machine-learning model that was trained on 306 of Bach's chorale harmonizations. Another team worked to allow machine learning to occur within the web browser instead of on its servers.