Dogs sniff things and spin in circles before they poop. All dogs do this. It's in their genes. They lick things. They smell food and go after it. It's written in their genetics; it's part of their physical programming.
Humans are the same, although we exhibit more complex behaviors, because our physical brain capacity is greater and we have a rich culture that has evolved over time and is taught to each new person throughout their life. It's like the evolution of genes, except instead of genes being the medium, our living brains and our environment (books, structures, theater, etc.) is the medium.
Free will, in the sense that there's some magical separate "decision entity" is completely bogus. It doesn't exist. There's no need for it. It wouldn't explain anything that isn't already completely explainable and obviously physically based.
As for how our choice of action originates; it's a complex system, but physically based nonetheless.
Depending on where we are at any given second, and given what we learned over our lifetime and what we expect to happen, and how those expectations interact with our immediate and long-term needs, we make choices. I am sitting here typing this, but I'm also thinking about my job and survival, and I have feelings of hunger, and knowledge about my daily routine, etc. I'm constantly making predictions, planning, and feeling things. Thanks to a highly consistent universe, such planning and predictions keep me alive. Partly written into my genes and partly learned from the environment.
There's always something going on in the body and the brain. Always, even while sleeping, as long as you're alive. I'm bombarded by advertisements, other people, sounds in the environment, my dog, airplanes, sunlight, clouds and shadows, the furnace kicking on, etc. Levels of chemicals rise and fall and shift and change according to genetic programs and physics. When I get so tired I can't sit here anymore, I'm compelled to go to sleep or risk going insane. Our environment drives us. Electromagnetic fields interact with our brains, as does everything we ingest, see, hear, and feel. Our choices affect us further in a feedback loop. The kind of free will you're thinking of simply does not exist. It doesn't even make sense to propose it. We are what we are.
That's one of the reasons I say that personal responsibility is a bogus concept, at least in the sense of blaming individuals for their actions, using them as scapegoats, when the true nature of problems are distributed in nature. When we blame individuals for their actions, we erroneously absolve everyone else from playing any part, and that's a mistake. We miss the opportunity to find the real sources of problems, like economic inequality, brain tumors, genetic predispositions, etc. It's just simple-minded to judge people for what they do without looking at the bigger picture.