Don't know if it's just me, but whenever I read or asked about quantum physics, I'd immediately get stuck on what it means that a photon is a particle and a wave, usually that exact phrasing. A wave of what? Some explanations even make it look like the particles move through space in a sine wave. And then they keep going on the math around waves, introducing the Schrödinger equation, and I still don't get what these waves physically are.
This doc is the first one I've seen that explains upfront what a wave means here: "The quantity which travels in this way is known as a 'disturbance'. Precisely what this disturbance is depends on the wave," and for EMR, "it is an electric field and a magnetic field which are perpendicular to each other and also perpendicular to the direction of the wave." So now I'm going to read the rest.
QM is a mathematical framework that started with "ok but what if matter does act like a wave" and then throwing the classical wave equation at the problem.
A lot of weird predictions pop out if you do that. Those predictions (where we can test them) turn out to be true.
This kicked off an argument about what exactly this means in a physical sense and a hundred years later, it's still going.