I think the first thing to accept is that we're 3-dimensionally bound beings, and we're still trying to figure out how to grasp the 4th dimension.

Best way to explain why 4D is tricky to grasp, is by going down a level in the comparison: pretend we're 2D beings trying to understand 3D.

If we're 2D beings, then that means we can perceive height and width, but not depth. That means you would be unable to see a sphere, but you would be able to see each 2D circle of it, if you were to slice the sphere into tiny cross sections.

Picture a sphere resting on a piece of paper. The flat circle it imprints on the paper would just be a dot. As you lower the sphere through the paper, the circle it leaves on the paper keeps getting bigger until you reach the middle of the sphere. At which point it will go back to being smaller again as you continue to lower it, eventually ending up back at the dot.

So while you can never actually see the entire sphere all at once for yourself, you would theoretically be able to infer the true nature of the sphere by stacking all these circles on top of each other. However the problem here is that you wouldn't even understand the concept of stacking the circles. The only thing you can perceive for yourself, is the 2D circle increasing and decreasing. It's difficult to try to imagine the sphere, when you've never seen a 3D object.

And it's even worse if a 1D being tries to grasp the sphere. They would only see a dot that grows into a line, which then shrinks back into the dot. To infer that you're looking at a sphere, you would have to take note of the fact that the line expands at a higher rate initially, only to eventually slow down in its growth until it stops. And actually, you don't have enough information available to you to actually conclude that it's a sphere. It could be a cylinder or cone lying on its side with the circle facing you, you'll never know.

So yeah, sorry if this seemed off-topic, but I just think the first thing to wrap your head around when trying to understand quantum physics is that our natural senses are limited. Things aren't supposed to make sense or feel intuitive, when you're punching a dimension above you.

To explain, quantum mechanics refer to the behaviors of particles, which work differently than large scale physics predicts. The most important aspect to this discussion is particles behaving differently when measured vs not measured. Light for example is a wave when not measured, and a particle when measured. The photons, before being measured, act as a wave becaue their values are not defined before measurement (or observation). This is the source of true randomness in the universe. Theoretically, you could predict the behaviors of anything in large scale physics by having the starting conditions. Quantum mechanics do not allow you to predict the future even with the starting conditions.

Since our brains are essentially quantum computers, it can not be claimed that our choices are pre-determined. This does not prove free will beyond a reasonable doubt, (randomness being involved in our decision does not entirely disprove the notion that our decision is just a mechanical process) but it is a very strong argument for free will because it at least contradicts the notion that everything that we choose is pre-determined by the starting conditions in a system.