Footage of Carrie Fisher and Mark Hamill from that movie, as well as The Empire Strikes Back, were used to digitally recreate the younger versions of Luke and Leia.
But it wasn’t just the ‘present day’ Leia we saw in Star Wars: The Rise of Skywalker.
We also saw a flashback of her training with her brother Luke not long after the events of Return of the Jedi.
We also already knew that Fisher’s daughter, Billie Lourd (who’s appeared in the Sequel Trilogy as Lieutenant Connix), stood in for her mother during that sequence.