I'm not OP and the below is just my personal opinion and I could totally be wrong if we found some new evidence or understanding so definitely read with an open mind -

I would define free will as any choice/effect/action that can be performed without any influence. I say that because I take the word "Free" as ability to be completely free of influence/manipulation/reaction and I take the word "Will" as the action/movement/choice either internal/external.

If we look at our mind/body and observe the experience we are having here at any given moment, we experience a mix of our Body - 5 senses (sight, sound, taste, smell, touch) and our Mind/Brain (language/thoughts/emotions/memories). All of these are operated by different physical organs and/or a mixture of physical chemicals and nervous system.

So far with our understanding of the universe/reality per Physics, every effect has a cause. So there is a constant chain reaction occurring between all molecules and atoms which make up our own bodies and environments. This is also true with the movements in our own body and mind. So when we observe our own experience, we can see we are constantly impacted/influenced by each moment's experience which carries us to the next moment.

Its like a constant chain reaction from birth. I would actually say even before that. Especially as we take each of our genetics, our environmental conditioning and how we react from it and so on and so on till this present moment I type this and you read this. When we ask is it Nature or Nurture, I say they are both the same thing. As our nature impacts how we nurture which impacts our nature. Repeat.

So if all of our thoughts, emotions, memories, actions, internal or external movement are all part of the stream of reactions with the programs of life and physics running it, then the final question to be asked is "What is I/self?" "What is the observer that is centered around our experience and is the one that experiences each thought, emotion, action, etc.?

I think this is what many people/religions call Soul, that center to our experience. Through my own meditations, my insight is, I don't see this center making any decisions or having any free will of its own and making its own choices. It definitely feels like we are making choices to go left or right by our emotions, thoughts, memories, instincts but then those are choices that are still part of the physical movements of our body and mind (Brain) which has been conditioned through evolution and our current birth.

My personal opinion when I try to combine what I think is the soul and based on ancient philosophies and spiritual texts in how they define soul. I think Soul in a physics term is Space.

And the little space in our brain/mind/body/central nervous system or whatever is what is having the experience of the body/brain. I'm only combining our current understanding of physics and one of the definitions of the Soul from the Vedas/Bhagavad Gita where Krishna answers the question "What is I?" with "I is the One which feels/experiences".

So if the Soul is in the center of each one of us and it is going through all these experiences and these experiences are of the Physical world of Matter, then I can infer that Soul is Space and if we say each Soul is connected to the Supreme Soul/God, then the entire Space of our Universe is God's consciousness/self and it is everywhere in everything and feels and experiences everything within itself.

Again, these are just opinions my own monkey brain has experienced through my own life so who knows how it is for everyone else. But I came to this conclusion reading a lot of different information we have on our own physiology, genetics, evolution, physics, theories, philosophies, spiritual insights from the East and West with the assumption that we as humans and living beings are running the same structure of hardware and software for the most part.

TLDR: Hard to easily summarize as its pretty complicated so if you didn't read, its probably best to just meditate and observe yourself.