This is the INJUSTICE 2 issue #6 we get to know about the origin of Supergirl.
As we move back the clock and see the early life of Kara Zor-el and how the destruction of Krypton occured.
She was sent to earth by much like Kal-el when she was young. Although due to an accident she arrived decades later.
She gets to know a completely different story of Superman’s regime by her benefactor who takes care of her.