They are thrown together from different DC comics, not even necessarily Supergirl/Superman ones
They tend to do this with the TV shows, Deathstroke for instance is a main player in the Arrow TV show but is primarily in Batman comics and Roulette from Season 2 of SG fought all the Justice Society of America. To make the continuity better for TV (I assume) plotlines in the comics are rarely wholesale implemented and more often there's a villain taken and then a separate story created around them.
Also the shows various characters have been modernised to include more POC and LGBTQ characters.