What Schrödinger’s cat REALLY means

What I don't get is why with QFT it is still so baffling, that quantum particles interact with the field. I get it, it's not like classical mediums like water but the QF is still the "medium" that underlies quantum fluctuations, right? The superposition is just not the classical stuff, that we are used to, but the probability of the particle's manifestation in a particular spot of that field, right? That was always my intuition about the experiment. And the measurement collapsing the wave function is also nothing mysterious. A measurement forces the probability of that particle to manifest or not manifest leaving a discrete probability. I know, that "If you think you understand quantum mechanics, you don't understand quantum mechanics" but in this case, I really fail to see the mind-bending.
And regarding Schrödinger's cat ... the detection of the decay is the collapse of the wave function thus again, leading to a discrete probability, not a superposition within the box. Us not knowing from the outside, if the particle decayed or not, is more an epistomological problem than a physical one.