i just watched derek's video[1], but there's something i didn't understand well.

to summarize, he talks about how the measurement of the schrodinger's cat leads to the entanglement of everything starting from who measured it. meaning there's a superposition of the you that saw the cat alive and the you that saw the cat dead.

so what he's saying is that as soon as the environment entangles with the contents of the box the universe 'splits'.
what i understand from this is that it splits because the contents of the box interacts with the outside.
but now suppose you have a box that doesn't let anything in nor out and you decide not to open it. you could say the cat that is alive knows it is, and so it knows it's in the universe where he didn't die; and sure enough both cats in superposition aren't just one entity so inside of the box the universe must've split. but what about the outside?
you could say it had split as well, but then it would've split when the atom decayed and not when the environment had interacted with the contents as derek said.

does any of you know where i'm getting things wrong?

[1]: https://youtu.be/kTXTPe3wahc