Schroedinger's cat is assumed to be simultaneously alive and dead in the box in which he is enclosed until the box is open. It is only then one can know the truth. Does that apply to everything?

Actually the story of Schroedingers cat was intended as a parody, highligting the silliness of Quantum theory at the time.

Even so it fits in many situations:

eg: Opening the oven door to see is the cake has been adequately baked (if it falls flat it has not)

The cat is probably meowing or clawing at the sides, so you would know it is alive. If it is some airtight or soundproof box, then the cat is probably dead from lack of oxygen and you should be charged with abuse to animals and thrown in jail.

Well, it does now...

In the strictest mathematical sense, yes, the principles of quantum mechanics are applicable at all scales.

However, the quantum weirdness which manifests at the smallest physical scales does not generally appear in the macroscopic physical world. Even the famous Schroedinger's Cat thought experiment is a bit misleading, because it includes a radiation detector which measures if a radioactive atom has decayed or not. The act of measurement creates a situation in quantum mechanics known as "collapse of the wave function." In layperson's terms, the mathematics goes from a set of probabalistic predictions to a definite result (i.e. the cat is either dead or alive).

What constitutes "a measurement" in the world of quantum mechanics is a bit of sticky philosophical issue, but in practical terms, there is enough "measuring" going on the Universe that we're pretty sure the Moon is still there when "nobody is looking at it" and Schroedinger's cat does NOT ever exist in a superposition of dead and alive states.

The alleged scientific method:
1. Observe something.
2. Formulate a hypothesis.
3. Devise a test.
4. If the test fails, go to #2.
5. If the test passes and is confirmed, the hypothesis might be promoted to a theory and used to prove other hypotheses. And it might not.

The actual scientific method:
1. Formulate a theory.
2. Make a computer simulation.
3. Compare the simulation to observed data.
4. If they don't agree, find some way to adjust the data. If you can't adjust the data, ignore it.
5. Be sure your fellow scientists will agree with your findings, then publish.

A big part of science is conjecture, which means making up "What if" scenarios. Often those scenarios get discussed a lot for a long time, and people begin to assume they are true just because they keep hearing them. I get a lot of insults for pointing this out, but that changes nothing. Any theory about something that has not been observed is FICTION.