Free will as in "there's something that can't be explained by chemistry and electrical signals in the brain" is just an attempt of some believers to rationalize their belief in the kind of the supernatural "soul."
The brain computes an abstraction of the reality (or most likely multiple competing abstractions) and decides on actions. There's the free will.
There is some kind of idea that the consciousness "floats" on top of this abstract soup as some kind of a running program in an operative system, and is making the calls, but this is most likely not how it works.
All these abstractions of sensory input, probabilities, decisions, etc, is probably what creates the consciousnesses but I doubt that there is a meaningful boundary.
After-the-fact rationalization may be true or not - it's obvious that events can't be processed in no time at all, and that there must be some processing time for the experienced "now."
But trying to build some meaningful philosophical arguments from it is probably a red herring.
After all, when trying to explain the reasons behind a decision made without explicit reasoning and external lists of pros and cons, it's often clear that there are many "hunches" that are not easy to summarize or valuate.
No one in "mainstream" neuroscience or cell biology takes these proposals seriously. Hameroff and Penrose have proposed some possible mechanisms (e.g., dendritic lamellar bodies), but none of these have panned out experimentally; for example, the DLBs are in the wrong place. They keep revising the theory around these data, but I think a lot of experimentalists have lost interest. I gather there are other more philosophical/first principles arguments against this too.
It would be neat if it were true though....
The deterministic effects in the brain must be negligible, as you can't reasonably reproduce the internal and external state of 100 billion neurons, swimming in a incomprehensibly complex chemical soup that is altered by the brain itself, at the same time experiencing the changing reality.