The psychological difference is really clear.
In the case of an involuntary action, the action comes straight from your unconscious mind to action, bypassing your consciousness.
An example of an extremely involuntary action, would be somebody taps your knee and your leg extends out suddenly.
This is very different from let's say you a driving a car and ponder whether to turn left/right.. pondering.. using your conscious mind.. / your conscious mind being engaged. And let's say you aren't battling extremely coersive unwanted thoughts that are coming from your unconscious telling you to drive off the road into a ditch. So you're pondering options.
An example of some Habits e.g. negative addictive habits, can be like that. Like a circuit in their brain gets triggered by some trigger.. and the action happens, whether you want to or not. You intend not to do the action, but you can't stop yourself. Like a person smoking cigarettes when they are trying to quit.. It's not as involuntary as the knee jerk, but it's far from a very voluntary action
Stress for example could cause more things to become involuntary. A lessening of self control.
On some level, all our thoughts are outside of our control. They come from our unconscious. The voice inside your head comes from your unconscious.
But there is a neurological and psychological difference between voluntary and involuntary actions. Particularly if comparing a completely involuntary action like the knee jerk.
Addiction examples that are not quite as involuntary could complicate things.
So just compare completely involuntary like the knee jerk, with actions you choose after freely and calmly pondering them.
That's the difference!
Don't take the terms voluntary action, and involuntary action, too literally.
Similarly the word control, or self control.
I wish Sam Harris discussed it more because telling people they don't have free will without telling them about their abilities of self control and voluntary actions..
He is so eloquent but maybe he doesn't discuss it much but he does believe in it. https://samharris.org/free-will-and-free-will/ "We can acknowledge the difference between voluntary and involuntary action"
Or as Dan Dennett writes https://core.ac.uk/download/pdf/228272025.pdf "Harris is a compatibilist about moral responsibility and the importance of the distinction between voluntary and involuntary actions"
(note, I don't know that sam is a compatibilist on moral responsibility.. or accepts moral responsibility.. and I don't know the ins and outs of when the term compatibilist applies.. but.. the point is, Sam does believe in the concept of voluntary and involuntary actions.. even though all is determined).
And compatibilists too believe in determinism..
The debate among determinists is over whether we define free will as magic. One could also look and say well do we define consciousness as magic.. Do we define "you" as magic..
Sam basically defines "free will" as magic.. But doesn't define consciousness as magic.
If we look at a definition of free will that doesn't involve magic / unscientific mumbo jumbo, then humans have free will.
If most people thought of consciousness as "the soul", then Sam might say oh most people think that, so then he'd argue that we don't have consciousness and it's an illusion. Though since Sam takes a non magical definition of consciousness he says it's a reality. And indeed it is a thing.. consciousness is a useful word.
Free will can be a useful word for a real thing but if one wants to do as Sam does and say that free will = magic.. the idea of our thoughts not being caused by our unconscious but having no cause or being their own cause. And the idea of a "you" like a soul, a ghost in the machine independent of the body independent of the brain.. That's why he says the self doesn't exist. He defines "self" as that kind of magic. He's being confusing really 'cos his audience don't think in terms of magic but he's talking to them like they do and he's not defining his terms clearly enough for people to know straight away what he's talking about.
We are fortunate that he accepts the definition of consciousness as not magical, so he uses it and writes very eloquently of thoughts arising into consciousness.
We are unfortunate that he doesn't write much about involuntary vs voluntary actions and how those work.. but he does accept them. If he were to take a non magical definition of free will aka "compatibilist free will" then we'd benefit from his beautiful eloquent descriptions of free will , sadly he chose not to! And he chose to not write much about the degree of freedom we do and don't have.
Sam does underestimate people to an extent for example he says that people have a false idea that they "could have done otherwise". But he's taking a magical view of "could have done otherwise". That if you "wind back the tape" by which he means if you go back in time, or have atom for atom everything the same.. Brain in exactly the same state. Then you couldn't have done otherwise. But people don't mean that when he claims they think they "could have done otherwise". Intelligent people mean.. If some thought had occurred to them that would not have been strange for it to have occurred. So if their brain had been marginally different e.g. on a different day. In a different moment. Or somebody very similar to them, with their ability their intelligence their kind of way of thinking, might have reasonably done something else. Like a clone of me that has lived an almost identical life, might have done something else in a particular moment. e.g. I could(reasonably) have had a different thing for breakfast today. Not to say that if all the atoms were arranged the same way in my brain that morning and in the surroundings.. then I would or might have done otherwise!