Free will is an illusion.
Everything is set because everything interacts in certain way; for proof, if something did not it wouldn't be interacting with anything. The ways something interacts is not changeable because is somethings properties cannot change it can't, and if something can its its property to be able to being the effect of a cause. The amount of directions exist, limits the amount of ways something can interact with something else and cause that object to move in a different direction. Therefore, everything has a prior cause which causes the present causes to occur, hence everything is set based off the prior causes.
randomness does not exist because everything needs a cause, because without a cause nothing is causing x to occur. Without that there is no reason for x to occur without the cause to occur before x.
You are right, but why can't the complex interactions of causes that result in decision-making and intentional action be called, 'free will?'
I.e. why do we even have the idea that some realm of possibilities exists that is 'free,' if the universe is deterministic? If we're not aware of how a given process is determined, e.g. because the mind's complexity is beyond our grasp/control, then by 'free,' we could actually mean, 'free of intentional control,' i.e. we cannot intentionally control the mind's processes of decision-making except by external coercion, etc.
It would be impossible for science/engineering to progress to the point where mental processes could be steered by external technologies in such a way that we experience our puppeted minds as if it were our own 'free will,' There is something about our experience of mental autonomy that makes us feel free, and it is probably because brain function is beyond external control.
So while 'free will' is determined by subconscious brain activity, we experience it as 'free' because it occurs autonomously. If you choose to abide the law, for example, you experience peace in that your decision to abide feels autonomous, but if you seek to break the law and experience stress of anticipation that you'll be caught and punished, then you experience your mental autonomy in a state of coercion and that steers you toward abiding the law; but of course as long as you perceive your free-will as being abridged by the threat of punishment, you will react emotionally against the coercion, and that emotional reaction impedes your experience of having freedom.