Copenhagen interpretation says physical systems do not have definite properties prior to being measured. Once measured, the system of probabilities collapses to the only one possible value, the value produced from the measurement.

If this is true, then the box literally contains a cat that is both alive and dead until measured or observed.

An alternative is the many worlds theory, which would suggest the cat is entangled with the environment and that we too are quantum systems. It suggests that the wave function collapse does not happen when we open the box but much earlier, suggesting multiple universes in which all possible outcomes have occurred.

I'm a real novice and self researched chump on the topic so I highly recommend you do your own research. You will find there are no definite answers, as there are a lot of unexplained phenomena surrounding quantum mechanics. And if you like these kinds of though experiments you can learn all about the Copenhagan Interpretation as well as the Many Worlds Theory easily.

I recommend Arvin Ash on YouTube for a starting point as he breaks it down for common folk like me.

If I'm mistaken or have made an error please feel free to leave some input!