In the movie Dark Phoenix, we see how Jean Grey got the Phoenix's power.
But in X-Men: Apocalypse, she used the Phoenix's power. Dark Phoenix can't be set before X-Men: Apocalypse since
Mystique died in Dark Phoenix.
Can someone explain how she used the Phoenix's power if she got it in Dark Phoenix?
It's not a retcon, because it's not said, in Apocalypse or Dark Phoenix, that this power we saw at the end of Apocalypse was the Phoenix Force.
Rather, it's a transition/climax scene of sorts which serves two purposes:
It's stated repeatedly in Apocalypse that Jean is even more powerful than she thinks, and that when she comes to realize it, she'll be able to do "whatever she sets her mind to".
That's what happens at the end of the movie: it's Jean embracing her powers, unleashing them upon Apocalypse, but really it's Jean's powers. Not an alternate personality, not yet a cosmic force, it's Jean. I went a bit more into details of this power and her self-control in my answer to How much self-control does Jean Grey have? on SFF.
Yes, absolutely it did. That's because Jean has what I call "Phoenix traits" - namely, while she wasn't the Phoenix yet, she shared some features with it, which is why she was chosen to be its receptacle.
To me it looks like the fire wings she got at the end of the movie were a callback to her astral avatar from the comics; from her entry on Marvel's website:
When Xavier introduced young Jean to the astral plane a part of her mind manifested as a Phoenix raptor.
Like I argued in an answer to Is The Phoenix part of Jean's mind or a separate entity? (still on SFF), combine Phoenix avatar + borderline infinite powers to begin with, and you get why Jean got chosen as the Phoenix's host (but once again, was not the Phoenix in Apocalypse).
Going by memory, Jessica Chastain's character in Dark Phoenix even said something along the lines of
Many people tried to master the Phoenix but never achieved it, but with you it's different, it chose you.
Note that the people who made Apocalypse already had their minds set on a Phoenix retelling, since Days of Future Past erased X-Men 3's events, allowing them to. For instance, in an interview with CinemaBlend, Kinberg and Singer certainly had planted that seed, but it wasn't the full-on Phoenix yet.
As to whether Bryan Singer is interested in finally getting to execute his vision for the Phoenix, he was ambivalent to answer. "You never know, but as far as the idea of that brewing within her, I could, without giving anything away, say absolutely, you may find a piece of that in this film."
This was what could be called a retcon
Reframing past events to serve a current plot need. The ideal retcon clarifies a question alluded to without adding excessive new questions. In its most basic form, this is any plot point that was not intended from the beginning. The most preferred use is where it contradicts nothing, even though it was changed later on.
But it's a very bad one...
So just ignore that part....at least that's what producers would like you to do!
The other issue with this is that the movie was plagued by a lot of problems....
Between multiple release date delays and several rounds of reshoots, the production of the latest X-Men movie Dark Phoenix wasn’t easy. According to stars Sophie Turner and Jessica Chastain, it also wasn’t well-planned, either.
Turner, the star behind the mutant Jean Grey who undergoes a terrifying transformation into her sinister alter-ego the Dark Phoenix, and Chastain, who plays the alien who manipulates Jean’s dark side, recently got to talking with Screen Rant about the numerous rewrites of Dark Phoenix. As it turns out, director Simon Kinberg was constantly making changes to the script on set
Dark Phoenix was just a finisher movie that nobody cared about and was made to just finish off the 'season' so to speak...and this is blatantly apparent, as you already noticed.
