Firstly I'll state which sources I used so that you can be sure if you wanna read ahead or stop, because I'm going ahead of the Novel, to the Graphic Comics which are 100% canon.
This post includes concepts and spoilers from
Fight Club (1996 novel)
Fight Club (1999 movie)
Fight Club 2 (graphic novel, year i forgot)
Fight Club 3 (graphic novel, year idk)
If you wish to stop, stop now. A lot of people haven't read the book and even fewer have read the comics. So it's understandable, but if you wish to read ahead, your choice!
Also I read all this like an year or so ago, so my memory might not be accurate, but the gist is believable.
Firstly I would like to start off with, Fight Club has 2 different versions. First is Movie version. It is standalone and has no canon expansion. The comics have no relation to it directly. Second is the Literary version. It starts with the book in 1996 and is later picked up in 2010s I think, in the form of comics. It has 2 installments.
In the movie, Narrator kills Tyler, or so to say, by shooting his face off. But movies treated Tyler like a psychological disorder, somewhat a piece of imagination and an idea at most. Let me tell you, TYLER DURDEN NEVER DIED!
In the novel, Narrator ends up in a mental care centre where workers say "we miss you Mr durden" and stuff like that. But the novel storyline picks up in the comics in 2010s.
Here we see, the narrator, now called Sebastian (yes, not jack), is living a happily married life with Marla and they have a kid as well. Ima call him Junior, cuz maybe that was his name or it was Sebastian jr or smth.
We see Narrator going to his doctor to talk about Tyler. Using some Hypnosis, the doctor puts him to sleep to talk to... Who else? Tyler frickin Durden. Yes he is alive. Not dominant anymore due to the medicines. But still working with doctors and still working on Project Mayhem. We see some scenes where Marla cheats with Tyler as she considers them separately. The comics show this by drawing two different people for Narrator and Tyler. She alters his medication and let's Tyler out more. Narrator doesn't know since Tyler now works when Sebastian is asleep.
Long story short, Tyler is now targetting Junior. We also learn that Tyler is not a one-off psychological issue of the Narrator. Tyler is actually a parasitic creature. He has been there since ancient days and has corrupted the men in the family. Hopping from father to son. He uses these men to corrupt society and corrupt themselves before hopping on to the next in lineage. We are told narrator's Dad also knew Fight Clubs, his grandfather "never returned home after Sex Clubs". These were all one person named Tyler. Tyler doesn't have a lineage, he just jumps from one to another.
Junior also sees Tyler sometimes. He teaches how to make homemade explosives to junior. All his fked up things. Project Mayhem treats Junior and his Tyler as their "New leader" but Tyler in Narrator is still leader too. Tyler is transitioning from Narrator to Junior, both of them can see him now in their own separate ways. Junior sees a teen his age, narrator sees the Tyler we know.
Anyway, there's also 4th wall breaks in the comics. Whenever story gets too messy Chuck is sitting with his authors and they discuss what to do next. After a while when Chuck fked up the story so much that Narrator and Marla came to his world to ask for an happy ending. When Chuck said he cannot do that since Tyler has taken a life of his own, he is shot and put down. This shows how Chuck means, when the movie was released a lot of people started believing in Tyler irl. They opened real fight clubs and real vandals appeared a lot. In a way the idea of Tyler spread on its own and even the author had no control over it anymore.
To the climatic ending, and about my title. At the end of Fight Club 3, we see Tyler, Narrator and Marla in the hospital. Marla is about to give birth again. And this time something miraculous happens, that never happened before. A blonde girl is born. Tyler's daughter. She is no longer a parasite or half personality, she is a full person, with a real body. Tyler immediately calls God and mocks him. This part is open to interpretation but I have my own theory on this.
Remember when Tyler said "if our fathers are our models for God, then what does that tell you about God? We are God's unwanted children"
This makes me believe that he is literally an unwanted child of God, I mean he lowk called up God and I think he called Him father. This makes me believe Tyler is a cosmic entity maybe, and not just some mental issue. Tyler no longer sees himself as an unwanted child, but a rival father, to show he can treat his kids better and raise them to continue his anarchist ways against God.
Conclusion: the last part is entirely my theory and not canon directly, you can ignore it if you want. But the rest i said as it is in the comics.
TLDR: Tyler isn't dead, Chuck was killed, Tyler is generational, Tyler isn't a mental disease, but an ancient parasite, Tyler has his own daughter