Kara tells Alex about Clark's friend Chloe and her Wall of Weird, a reference to the Smallville (2001) character Chloe Sullivan as played by Allison Mack. In 2019, "Smallville" was retroactively connected to the Arrowverse when Tom Welling and Erica Durance played their versions of Clark and Lois in the "Crisis on Infinite Earths" crossover, which also retroactively suggests here that a version of Chloe exists on Earth-38.
This flashback story was created in part to allow Melissa Benoist (Kara Danvers / Supergirl) and Chyler Leigh (Alex Danvers) the time needed to appear in the four part Arrowverse crossover: Crisis on Earth-X, Part 1 (2017), Crisis on Earth-X, Part 2 (2017), Crisis on Earth-X, Part 3 (2017) and Crisis on Earth-X, Part 4 (2017).
Vancouver Technical Secondary School is used as Midvale High School. The same school was previously used as Smallville High School in Smallville (2001).
In the comic books, Midvale was named because its location was midway between Smallville (Clark Kent's home town) and Metropolis.
This is the first episode of the series in which Jeremy Jordan (Winn Schott) does not appear. As such, Melissa Benoist (Kara Danvers / Supergirl) is the only actor to appear in every episode.