Josie Campbell did an interview on the film.
Main points:
Wrote script in 2020, borrowed Krieg's LOSH trades, had seen Man of Tomorrow when she wrote, production team sent her links to others that were being worked on at the time, teen/high school/college romp feel - people Supergirl's age figuring things out with each other, is through the lens of Supergirl, had this plan for her life then her world was ripped away from her, Kara's agency mattered when she wrote it, trying to prove she belongs on Earth. Latter half is about Campbell's background.