For the machine learning enthusiasts, here is an old article on how Spotify classifies songs into genres based on what they actually sound like (input is a spectrogram) https://outline.com/C3PVuU.

Most conventional methods used to recommend based on what other artists people with the same taste as you like. This method is biased against newer artists as there is no easy way to bootstrap them into the process. Their new technique circumvents this problem and as a result new artists get more exposure.