The song playing at the end is an English cover of the song "Hymne a l'amour" by French singer Edith Piaf.
Actress Cailey Fleming may look familiar to sci-fi fans, Sylvie is not the only character she has played a younger version of. Her first theatrical appearance was as the young Rey in Star Wars: Episode VII - The Force Awakens (2015). This footage was reused in Star Wars: Episode IX - The Rise of Skywalker (2019). She has also appeared on Preacher (2016) as Susie (another series based off a comic). Most notably she plays Judith Grimes on the AMC series The Walking Dead (2010) (yet another series based on a comic).
The song playing at the end is If You Love Me (Really Love Me) sung by Brenda Lee from the album Emotions.
The engravings on the entrance door to the Time Keepers chamber are in the shape of an hourglass, which appropriately symbolize the passage of time.