Some of these pictures and descriptions may give away plot details that you might not want to know before watching the film.
This is a Steven Spielberg-directed adaptation of the 1957 musical (made into a film in 1961) about Maria (Rachel Zegler) and Tony (Alsel Elgort) from rival gangs in New York City, who fall in love, even though it is forbidden. It's loosely based on Shakespeare's Romeo and Juliet.
There are many scenes with mirrors in the movie. In one, we see Maria and her sister Anita (Ariana DeBose) in front of dressing mirrors getting ready for a date, beginning at approximately 27:30.
Maria and the cleaning crew she works with go to a large department store, where Maria starts proclaiming her love for Tony as she puts on a sash and pretends to be Miss America. She sings the song "I Feel Pretty" in front of a room full of dressing mirrors as her co-workers and their boss sing along. Finally the boss tells them all to get back to work.
The dressing mirrors part begins at approximately 1:50:30.