The "free will" as it is used by those who insist on it existing is something that is based on the needs of religious thought. It's a concept invented to explain the problem of (paraphrasing) "if good is all-powerful and good, why is there evil," to which the magical answer is "the people have free will" to mean, the God doesn't decide what people will decide. And this is just nonsense invented to explain the starting nonsense problem in the "demon haunted world"(1) of those who believe in a "good God."(2)
Our decisions are simply the function of the "stored state" in our body and outside influences in the moment up to the decision. Animals function the same. Of course, there are so many influences and the process of thinking(3) is dependent on so many variables that we can actually make different decisions based on, for example, our same experiences up to the moment of the decision but depending on if it's 9 AM or 9 PM, or if we drunk the coffee one or two hours before etc. So it appears to be a "free" decision but is still just a function of the state(4) and the input.
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1) https://en.wikipedia.org/wiki/The_Demon-Haunted_World
2) No every religion has a "good for everybody" god, so for these religions it's not even a problem. If the god is there just to favor your group when he is not distracted by something else (yes, there are religions where the god has a limited attention span), he can smite everybody else and he doesn't have to be absolutely "good." For those who believe in a "good God" the shock is, of course, when something like this happens: https://en.wikipedia.org/wiki/1755_Lisbon_earthquake#Effect_...
3) which we surely believe to posses, in some aspects more complex than what we understand the animals do, simply because we don't know their language and they can't write
4) not just of our neurons but also of the exact amount of the traces of different chemicals