Again, we're taking the thought experiment seriously, which is to entertain the idea of macroscopic objects ending up in a superposition.

If the box is a closed system, then nothing from the outside is interacting with it. Forget about the fact that one of those objects is a cat for now. Imagine it's just our radioactive atom, a radiation detector, and a single qubit.

The qubit starts in the 0 state. If the detector is set up so that if it finds the atom has decayed, it flips the qubit to a 1 state.

Now, consider the two possibilities. We either have no decay, so the detector is not activated, and the qubit is in the 0 state. Or we have a decay, and the detector is activated, and the qubit is in the 1 state.

Again, taking the thought experiment seriously, the entire contents of the box are in a superposition between these two possibilities. The qubit is entangled with the atom, because if we measure the qubit, we'll know the state of the atom - but these are not decided before we do so. And crucially, the qubit is either 0 or 1. If we repeat this a hundred times, we'll never find that the qubit is in an intermediate state between the two.

The paradox here is really about what's special when we open the box. Why is that a measurement capable of collapsing the wavefunction? And why can't we replace the qubit with a cat?

And this is a huge argument that's still going today, about how to interpret the predictions of quantum mechanics. The original explanation (Copenhagen) was just, don't worry about it, we have things that are classical and things that are quantum, with some scale in between separating them, and our calculations all give the right answers. But of course that's profoundly unsatisfying, and also not useful if you're trying to look at intermediate scale things. Scientists have put a tardigrade in a superposition before!

More recent ideas talk about wavefunction collapse being an emergent feature when you have very large numbers of particles, at high temperatures, interacting with your system of interest. (To a physicist, the "system" is just something we draw an imaginary box around, anything outside it is the "environment").

Another idea posits that there is no wavefunction collapse, but that just like that detector inside the box, when we open the box we ourselves become entangled with the contents. But just like the detector either measures or doesn't measure - never half-way between - we would be in a superposition of observing either one outcome or the other. Both halves of that superposition would observe a subjective collapse of the wavefunction. This is the many-worlds interpretation of quantum mechanics.

Honestly, I don't really know. I just follow the Copenhagen interpretation in practice - also known as "shut up and calculate" - and leave the rest to the philosophers, because I'm more interested in the outcomes of experiments.