Today Spotify has taken away some API endpoints, leaving many users in the lurch. ListenBrainz has been working hard to create similar API endpoints and datasets, all freely available.
Your profile makes it clear this is your project. I recommend also stating
this explicitly in your comments, as your username doesn't suggest a
connection.
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.
Edit. Found this (app list linked here as well): https://musicbrainz.org/doc/Beginners_Guide