
I get the story but I don't at the same time...

Basically the omegaverse world is the real world. The contents of Silence, the novel, really happened. They really did die together. However, whole he was dying, Huimin actually managed to mainfest a fake reality (sort of like a pocket dimension) where he could live out his dream life. And because he still cared very deeply for Iheon, Huimin ended up dragging Iheon's soul into the fake reality as well. But because the world is not Iheon's, Iheon had to play by that world's rules and was prevented from altering or interfering with its "truths".
For example, in Huimin's fake world, Iheon is Huimin's best friend. That is a truth. Iheon tried changing that truth and professed his love but the fake world would interfere and block Huimin from hearing and seeing what Iheon said to him. Quite literally pressed the mute button on him lol
Iheon eventually realizes that this fake world is prewritten or predrawn. Huimin has dictated how things will go and these events will happen, regardless of what Iheon tries to do. However, Huimin's drawings, his stort, is finite. There is an end. And when they were close to the end, Iheon noticed the fake world was crumbling away. He therefore hypothesizes that once the last event that Huimin has drawn happens, he and Huimin's souls or consciousness will cease to exist. They'll die for good.
For Iheon, this is unacceptable. If he can't have Huimin in this fake world then he'll drag him back to the real world, even if it means death, because then at least the truth is they were lovers and not just friends. So Iheon went to Huimin's fake-parents' house while time was still stopped there and placed the book Silence in Huimin's room. When Huimin went home for vacation, he finds the book and reads it (the beginning of this manhwa). This somehow becomes the catalyst for the fake world breaking apart and their souls/consciousnesses are sent back to the real world.
However, in a lucky twist of fate for Iheon, they're sent back in time to the beginning of when they met again as adults in the real world. However, Huimin was still stuck as the Huimin in the fake world so Iheon decided to do things differently this time. He behaved somewhat the same but he was more careful and "loving" towards Huimin.
That's it??? Now give me the side storiess pleasee