Artist similarity is calculated from the listening history of all users. Basically if many people listen to artist X and artist Y within a short window of time, that counts towards their similarity score. Also artists that appear together on compilation albums also counts for similarity, but to a lesser degree.

In the near future we want to add more data from MusicBrainz, for an easy example band members being part of other bands = they are probably similar.

All of this is available on ListenBrainz, but will most likely not be surfaced on the MusicBrainz pages which have a different —more factual— focus.