(at around 46 mins) According to writer Eric Pearson, Florence Pugh teased Scarlett Johansson about Black Widow's iconic pose during filming, so Pearson put it in the film.
(at around 53 mins) In prison, Red Guardian shows a tattoo on his right shoulder of two roses. The two Russian names underneath them translate to Natasha and Yelena.
In the movie, Alexei Shostakov (Red Guardian) is a father-figure to Natasha Romanoff (Black Widow). In the original comics, Alexei and Natasha were husband and wife.
On July 6, 2020, it was announced that Marvel Studios executives, including Kevin Feige, were so impressed with Florence Pugh's performance as Yelena Belova that they plan to include her in future MCU films, and she may even become the new Black Widow. In fact, Feige stated that this film was written to showcase Belova just as much as Scarlett Johansson's Natasha Romanoff.
Ever Anderson secured the role as young Natasha because she already speaks Russian and has a green belt in taekwondo.