Question: are supergirl and the flash on the same earth?
Answer: In this new universe, as revealed by the Legends of Tomorrow episode, all of the CW's superhero shows — Arrow, The Flash, Supergirl, DC's Legends of Tomorrow, Batwoman, and Black Lightning — now reside on the same planet, which is called Earth-Prime. ... So all of the CW [superhero] shows will be on the same Earth.”