Force ghosts were not just able to visually manifest themselves anywhere in the galaxy. They were also capable of interacting with living beings, physically, and they could manipulate the Force.
The Force ghost was the soul and essence of a deceased Force-sensitive who denied the will of the Force upon death, yet was able to interact with the living.
Force spirits were not only able to appear anywhere in the galaxy with a strong presence in the Force, and some were able to even manipulate the Force to similar extents as they could in life.
Force ghosts only appearing to Force-users makes sense on a thematic level. The ghosts are said to comprise entirely of Force energy - sentient manifestations of the mysterious, unseen energy the Jedi wield. It's logical that only those who can use the Force themselves can see the spirits it creates.
However, Palpatine was never able to achieve the immortality that the Jedi possess. His plans brought him closer than any other Sith to becoming a Force Ghost. But his failure proves that so long as the darkness outweighs the light, the Dark Side will never lead to immortality.
Qui-Gon Jinn: The Greatest & Wisest Jedi of his Time (Canon Fodder Discussion)
Why isn't Mace Windu a Force ghost?
Why didn't Mace Windu showup as a force ghost? Qui Gon died before he could finish his training so he can only appear as a voice and not a physical form. Mace Windu never studied or trained in that aspect of the force.
The closest that Star Wars canon has come to naming who trained Yoda was revealing that Jedi Master Qui-Gon Ginn was the one who taught Yoda to become a Force Ghost and attain immortality.
Qui-Gon Jinn learned to become a Force Ghost by training with Force Priestesses and mastering the ability to appear after death. Jedi must be fully committed to the light side to perceive Force Ghosts, while the Sith cannot become true Force Ghosts but can cheat death.
There is a movie theories video say about this. Basically that Force Ghost can only exist in a enviroment that rich in life ( so also rich in midichlorian), so they can't go to place like Death Star, Cloud City or Palpatine's Throne room.
For those who use the corrupted dark side of the Force like Sith Lords, returning as a spirit is dangerous, violent, and chaotic, often restricted to a specific place or object. Fascinatingly, beings who aren't Force-sensitive have an afterlife in the Force, but they cannot return as ghosts.
In material that is now considered Legends, Qui-Gon is even described as a Gray Jedi. Additionally, the term has also been applied to Force-sensitives who aren't affiliated with the Light or Dark side of the Force. Quite literally, these Gray Jedi don't see things in black and white, or light versus dark.
Becoming a Force Ghost was a lost art that Qui-Gon rediscovered. Qui-Gon never completed the process, which is why he only ever appears as voice instead of a full ghost, but he taught it to Yoda and Yoda taught it to Obi-wan.
When she awoke, they kissed, and Ben fell over dead. Then, he faded into the Force, and Rey was left alone. The fact that Ben faded into the Force seemed to indicate that he became a Force ghost. After all, every Jedi that has faded in the past has retained their consciousness.
Summary. The ending of The Rise of Skywalker sees Leia as a Force ghost, but it's never explained how she was trained to do this after her death. Leia's ability to become a Force ghost isn't far-fetched, as she had three Jedi Masters who could have taught her - Luke, Obi-Wan, and Yoda.
However, Grogu was captured by Imperial forces, and Djarin was forced to lead a mission to rescue him. After being reunited with Grogu, Djarin delivered him to Luke Skywalker, a Jedi who agreed to take the child into his care and train him in the ways of the Force.
Force Ghosts only speak when a Jedi needs guidance, and since Anakin didn't say anything to Ahsoka, she no longer needed his help. It represents Anakin passing the torch to Ahsoka, as his story received closure thanks to their meeting in the World Between Worlds.
We are led to assume this as in TCW Qui-gon admits his training was incomplete. He could only appear on Mortis, anywhere else he could only be a disembodied voice. Wait, if he hasn't completed the training, how did Obi-Wan?
This implies that Force ghosts are not actually immortal, in Legends at least. While Obi-Wan's Force ghost faded away after 9 years, Luke's Force ghost appears to Cade Skywalker much longer after his death.
Ok so let's get into it Mace Windu survives, The fall but he has lost his weapon and is servilely wounded in now hostile territory. This is why I believe he would go into hiding for some time thoughout the early years of The Galactic Empire.
Summary. Rey received guidance from the voices of past Jedi, including Obi-Wan Kenobi and Anakin Skywalker, who encourage her to rise and defeat the embodiment of evil. Ahsoka Tano's voice reached Rey through the Force, suggesting she must have rejoined the rankes of the Jedi before her death.
...was redeemed at the end like Vader was. We will see a force ghost of Maul who will be trained by Qui-Gon to be able to live forever in the Force. He accepted his defeat and the Dark Side died inside of him in his last moments as Kenobi held him.
