Time loop is something really hard to understand, especially since the past influences the future and the future influences the past kinda thing. So I wanna try to break it down, how would it flow if we see how the time loop started?

Canon Event:
- Chen Yu Ru's death and Mo Jun Jie's arrest in 1999
- Mo Jun Jie's death in 2008

How the body possession works.
- When Li Zi Wei possessed the body of Wang Quan Shen, both of them were in mortal danger. Wang Quan Shen's soul should be dead, while Li Zi Wei's soul might have split into two bodies. That's why they can both live in the same timeline.
- When Huang Yu Xuan possessed the body of Chen Yu Ru, only Chen Yu Ru was in mortal danger. Chen Yu Ru's soul was still alive but trapped in the mirror's room, that's why the timeline stopped when Huang Yu Xuan went back to the present.
- When Xie Zhi Qi possesses the body of his brother, neither was in mortal danger. That's why Xie Zhi Rong can still influence his body since his soul (to some extent) is not weakened or not trapped in the mirror's room. 

1. 1st time loop - started by Li Zi Wei - 2003 to 2010
The first-time traveler must be Li Zi Wei since he is the one who possessed the tape player. At the time, he had just visited Mo Jun Jie in prison, then he had a car accident. When he time-traveled to the body of Wang Quan Shen, it was already 2010 and Mo Jun Jie is already dead. If we assume it's his first time traveling, then he wouldn't know that he will go back to his body after Wang Quan Shen's body dies.
Maybe he earnestly tried to live as Wang Quan Shen and somehow he meets Huang Yu Xuan, of course not as early as we see in the drama. Because Wang Quan Shen is originally from a different city, why would he go to the city Huang Yu Xuan in without any reason?
After his first death as Wang Quan Sheng, he goes back to another 2003, and now he has the chance to save Mo Jun Jie. Of course, Mo Jun Jie will still die in 2008 since the root of his suicide is Chen Yu Ru. When it's time for Wang Quan Shen to die in a plane crash, Li Zi Wei must have met him to give him a chance to die so he can try to save Mo Jun Jie or continue living for the sake of Huang Yu Xuan. Maybe, for Li Zi Wei/Wang Quan Sheng, Mo Jun Jie's death is a knot in their heart. So Wang Quan Sheng chose to die so he can return to 2003.
After this cycle happens many times, there may be a singularity, which happened in the drama, that made Li Zi Wei send the tape to Huang Yu Xuan. 

2. 2nd time loop - started by Huang Yu Xuan - 2019 to 1999
Because Li Zi Wei fell in love with Huang Yu Xuan inside Chen Yu Ru's body, in the future he met Huang Yu Xuan earlier (2010). Her possessing Chen Yu Ru's body changed the outcome, since it caused Xie Zhi Qi to time travel, which then caused the tape to be partially broken. Because of that, Huang Yu Xuan was able to talk to Chen Yu Ru and tell her to keep living

3. 3rd time loop - started by Xie Zhi Qi - 2019 to 1999
I think his brother Xie Zhi Rong is indeed obsessed with Chen Yu Ru somehow, but he isn't that twisted to try to kill her. Xie Zhi Qi might have some problem with his mentality initially. I think in the original timeline after he killed his pet, his brother originally was shocked and chastised him that's why I feel he doesn't really love his brother at least just because they're the same type (since he put him in a mental hospital). So when he went back he tried to satisfy what he expected from his brother.

When the tape was broken, all time loop got broken. But the crux of all time loop which is Chen Yu Ru's death is solved since she won't commit suicide anymore. 

The thing with time loop is that you have to turn  to a blind eye :) 

 David Sun:

There's a plot outline summary in wiki page.

https://en.wikipedia.org/wiki/Someday_or_One_Day#Plot_outline_summary

Actually I want to understand the motif of event and action rather than just the timing ahahaha