Ok, so listen, this review is my personal opinion, don't give hate to me neither the people who made the anime.

So you know, the anime was great and all but, the people who haven't read the manga before watching the anime, like me, I was confused like "Who is this? Where is Kaneki?", so after Haise encounters Niseki and just starts going beserk and little scenes of Kaneki were being shown just like in Tokyo Ghoul's 1st season, I though that he died and his 'Kidneys' were given to another person but then again like in the ending of the anime he turned into black haired Kaneki, I then watched some clips of the iconic chessboard scene of Haise turning into Kaneki and when I read the comments there was this there were multiples that were explaining this confusing parts :

'If anyones actually confused why theres two of him, i'll explain as much as i can and do it as simple as i can, okay first of all kaneki suffers from a personality disorder or a mental health that lets him have two or more personalities for him he only has two personalities, and that is kaneki/haise on the first season he only had one personality and that is kaneki the day that his friend technically died he caried hide's body and gave it to the ccg and after than it ends and here it transfers to tokyo ghoul:re, and this is where haise was born hes other personality, (to let you know personality disorder is one of the most deadliest disorders in the world)
And here in tokyo ghoul:re this his other personality called haise, the fact that this is not hes actuall personality is a problem since he suffers from it the moment he realised back in a few eps that he killed a ccg really triggered hes other personality to be remembered and that resulted in this flashbacks of him seeing his past self, but he denies it because he thinks this is who he is and that is (haise) but since thats only hes other personality he will never forget kaneki because again that is his true personality which he cannot forget, leading him so even more suffering, but later on in the anime he finally accepts his denied true self and decides to work with his copy kaneki and finally returns to his original self again and then he left ccg to be with touka which resulted in him destroying whole of tokyo, but since he realised and accepted his true self he continues with the final fight and lives with touka and had a baby, the ending was pretty wholesome i suppose but the ending really just was him accepting who he was and again the final fight.

NOTE: i am not a expert when it comes to mental disorder, this is only based on research i did none of this is actually proven since again im not a professional or an expert, this also has some guesses so expect some kind of inaccuracy
(im sorry if i missed something dont attack me for it)

Edit: in short terms it all started with fight of arima i think the fight with arima caused kaneki to be someone else and that is haise, or so i was told. This is the main source of kaneki to haise and from innocent kaneki to ghoul kaneki was from the torture from yamori'