i think the premise is that like, the world is unfair, and in this universe people's resentment about the world congeals into these "black rooms". And through these rooms people can like "unlock" their malice which allows them to live happily by taking their happiness from other people (i think generally through wish fulfilment).
Thats what the hands in the mirror are, theyre yeonwoo's malice which he can use to "evaporate" people.
Also because they've like unlocked their malice together trying to ignore that side of them or even like being apart leaves them feeling incomplete, which is why jun feels really lonely and on edge early on. I think it's also meant to be why he's so nice, because he's like suppressed his malice to the point he forgot his memories about yeonwoo, so he feels he can't retaliate at people. Its also why yeonwoo is so particular about jun i think, because their malice like "merged" (at least i think that's what's implied idk i didnt write ts)
bruh can someone explain ts whole story to me, I feel so dumb. (you can spoil)