Spoilers!!
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I'm assuming you're caught up? So this is my interpretation. There's 3 timelines happening, in 2 of the timelines, either ML died or MC died and the world's for those basically ended. In those 2, time was reversed (I think) to try and save the world (probably with the help of coco's owner, only remember the chickens' name rip). In the 2nd timeline (I think), MC was alive, ML dead, MC used themselves (and their watch?) as the fuel (??) for the 2nd time reversal (which creates the 3rd timeline). In the memory dungeon we see, created by coco's owner, MC is experiencing a world that was requested by 2nd timeline MC as an escape from his life where ML has died and the world is doomed as a "rest area" he can go back to when he's risking his life. OUR mc (3rd timeline), now has to figure out what happened in the previous timelines so that he can save his own timeline (which will be the last time that time can be reversed, I'm assuming). This is all from memory so I'm sorry if it's a bit confusing or slightly wrong oop
Okay so first timeline Lee Sayoung dies so J goes insane and asked hong yeseong to create a method to turn back time, J gave Hong Yeseong his watch which then HYS turned into a time machine of some sort.
When turning back time the previous timeline would merge with the new one and disappeared, but someone has to hold the weight of a ruined timeline, that person is called pivot of time. J is that person in the first timeline (spoiler it is a very heavy price one of the most tragic moments in the novel).
In second timeline without spoiling too much of the novel J is unable to handle the weight of the fallen timeline 1 so something happens which leads to his death.
After his death 2 timeline Sayoung takes J's watch and turns back time to bring him back to life, but instead of the timeline disappearing it remains held by the second pivot of time that is 2tl Lee Sayoung the reason is a spoiler so I won't explain much, (it is a tragedy, my baby LSY). ┗( T﹏T )┛
Now in our third timeline, both don't remember the other tls, but as a consequence of turning back time the west sea rift appears which J gets stuck there for 8 years.
Now in this arc, J first see the memories of the first tl in which LSY isn't awakened and then later dies.
In the last chapter 3tl LSY (the one we know and love) remembers that J dies in the second tl which is why he was panicking leading to the kiss they shared. And 3tl J realized that 1tls LSY died which also influenced him to kiss LSY.
Now they are stuck in the 2tl trying to find the way back to 3tl.
The man in black coat is LSY of the second timeline, he helps Ga-eul and leads her to a place near her timeline LSY and J

Someone please explain what is happening with the plot my tiny brain is really confused here