Created by Greg Berlanti, the Arrowverse started as one small Arrow/Flash universe, and has now become a franchise taking place on Earth-1. Shows like Supergirl and Black Lightning are not canon to the Arrow/Flash universe because they didn't originally air on the CW. After Crisis on Infinite Earths (note: this link leads to the comic), they both take place on Earth-Prime, but it's almost exactly like Earth-1 due to the Spectre.
Shows included in this post-Crisis Earth-Prime are:
Notable Parallel Earths from Pre-Crisis: