The young mother character is played by IU, who is a major pop star known as IU in South Korea. Director Hirokazu Koreeda cast her because he was impressed by her performance in Mi Señor (2018), a Korean show he watched during quarantine.
One of the two police officers notices the song "Wise Up" by Aimee Mann and tells her colleague that they heard the song in a movie. The movie is most likely Magnolia (1999), for which the song was originally written. That film also dealt with strained parent-child relationships. Also when the song plays in Magnolia, Julianne Moore is sitting alone in her car in the rain, which is what Bae Doona is doing in Broker.
While preparing for De tal padre, tal hijo (2013), director Hirokazu Koreeda discovered the similarities between the Japanese and South Korean adoption systems. He also learned about the baby box system: in Japan, there is only one baby box, but it's much more present in South Korea. He discussed the ideas with three Korean actors: Song Kang-ho (who plays Sang-hyeon, the laundry owner), Gang Dong-won (who plays Dong-soo, Sang's right-hand man), and Bae Doona (who plays Soo-jin, the detective), whom he had already met at festivals and film shoots, and then decided to combine the two ideas into one movie.
Second foreign-language movie for Japanese director Hirokazu Koreeda, after La Verdad (2019), which was shot in French.
The film started as a 4 or 5-page treatment called "Cradle." Then when the script was written, its working title was "Baby, Box, Broker," but writer/director Hirokazu Koreeda ultimately settled on "Broker," as he realized that the 'broker' in the film changes as the movie progresses, which was a very appealing concept to him. It's also a very simple and strong title.