Hi!  This hasn’t been addressed at all, unfortunately.  The movie doesn’t cover it and the novelization doesn’t do anything to address it, either, and is probably even less clear on it than the movie.

It’s ambiguous enough that it could be anything, it could be echoes from the past and the other Jedi aren’t really maintaining their spirits in the Force because they didn’t have the training, that the Force is touching Rey to moments of their lives, since time is not linear in Star Wars.  (Unlikely since they call her by name, but I’m not entirely discounting the possibility.)

Or it could be something that they haven’t written into this yet, like Anakin being the literal Force baby that he is, dragged them all back into consciousness after he got there, scooped up their essences and balled them back into themselves. 

I actually am incredibly partial to this idea, especially because “Anakin had help from the other side” is basically George Lucas’ way of (possibly) saying, “Yeah, Obi-Wan totally cheated him through the process.” which then can be used for me to go, “Like Anakin Skywalker wasn’t going to god-modding his way through the Force, OF COURSE HE WAS.” when he helped Luke over, who then helped Leia over, who then both helped Ben over.  (In the novelization, he has a line that pretty much solidifies him as a Force ghost, even if he can’t yet manifest:  “A voice came to her through the Force, clear and strong. I will always be with you, Ben said.”)

Or tt could be the Force bringing them forward one last time for its second Chosen One, gathering them up just long enough for their voices to reach Rey, before they’ll disappear again and they can never manifest themselves like Obi-Wan, Yoda, Anakin, Luke, and Leia can.

While there is the line, “The battle above disappeared. Instead, Rey saw a perfect sky, vast with stars. Peaceful. Light-filled. It was like she was staring through a window to somewhere else, a place between places.” when Rey is looking up at the stars over Exegol (and there’s implications in the Visual Dictionary that might play into the idea), I don’t think it’s hard canon that this is the World Between Worlds where the Force Ghosts are.  When Ezra walked through there, he was hearing lines that were specifically from other points in time, while the characters here are saying brand new lines.  (Also, that’s not really how the WBW works, if something comes through it to a time different from its own, it’ll go against the natural order of things.)

I thought I’d be cranky about the idea, because Rebels showed us specifically that Kanan faded into the Force, that he was gone, but I dunno, I feel like if anyone could gather them back up in a really awesome moment, it’d be Anakin Skywalker.

And I may be incredibly touched by the idea that, as the one who killed the most Jedi, he’s the one who is now bringing them back, that he has truly come home to the Jedi.