Should I read the novel to understand??!!

Yes. Read the novel, this manhwa didn't explain the plot well. It was rushed as you can see. The novel explained it much better.

Girl let me tell you the amount of times I had people on here explain it to me, and somehow the manhwa tied it up and now I get it (I hope) ಥ‿ಥ

So essentially, our MC that went into the game—that is his actual world. The MC Doppelganger is from the real world. When they were young they switched places. So our MC grew up in the real world, when he shouldn't have.

So when he woke up in the game world, he was finally back to his own world. The doppelganger returned to his,—the real world. That's why we see why the doppelganger MC going to an interview in the real world and bumping into the ML doppelganger who has no idea who doppelganger MC is.