How were the scenes featuring Princess Leia in The Rise of Skywalker filmed?

Question

As we know, our beloved princess, Carrie Fisher, died before the script for The Rise of Skywalker was even finished. According to this answer, digitally recreating her image was out of the question.

Iger and Bay also noted Fisher’s scenes in “The Last Jedi” wouldn’t be modified using hologram technology. Disney had used a digital re-creation of a late actor in a previous “Star Wars” film.

“We’re not doing that with Carrie,” Iger said, describing the upcoming film as “just as satisfying” as the last “Stars Wars.”

So how were the scenes featuring Princess Leia filmed?

Answer 1

The relevant info can be found in ScreenRant's How Star Wars 9 Brought Leia Back & Which Scenes Were Actually Carrie Fisher, a link which is also provided in the short answer on Movies & TV SE.

Mostly reused footage from previous films ...

In order to bring General Leia back, J. J. Abrams used recycled footage, most of which came from only eight minutes of unused film from The Force Awakens. Abrams also reportedly used some footage from The Last Jedi, according to Carrie Fisher's brother Todd, although the exact amount of film that was available to him is unclear. This meant that while Abrams had quality and original footage of Fisher playing General Leia, he was greatly limited in his options. In order to develop Leia's role in the story, Abrams had to work backwards. First, he and his team analyzed the footage that they had available, and then they reverse-constructed Leia's role based on what was available to them.

... with some special effects for the visual parts ...

While Abrams had to plan Leia's role around what Fisher said and did in the footage that he had, the special effects team was able to alter the physical appearance around Fisher's face to make a coherent story. Because the special features of The Force Awakens includes some deleted scenes, some of the footage used in The Rise of Skywalker can be seen in its original format. By comparing The Force Awakens with The Rise of Skywalker, it is possible to see just how much Carrie Fisher's footage has been altered to make her fit into a new role in a new story. Everything is changed, from the background to who Leia is speaking to and Leia's clothes and hair.

... and a couple of body doubles.

• As Leia lies down, trying to contact Kylo Ren, she is shown in silouette; presumably a body double is used for these shots which are specific to the plot of The Rise of Skywalker.

• When Luke's Force Ghost gives Rey Leia's lightsaber on Ahch-To, there is a brief flashback that shows Leia completing her Jedi training. [...] In this short scene, stunt doubles are used for both young Luke and Leia. The technology used is likely similar to that of Rogue One that recreated the face of a young Princess Leia on a body double.

• Leia's Force ghost, alongside Luke's, appears in the closing moments of the film. Leia's translucent blue form is not de-aged, and she is dressed in a brilliant white robe with a cloth draped over her hair. While Leia's face uses recycled footage from The Force Awakens, her clothes were likely edited for Star Wars: The Rise of Skywalker.