Well...
...in Tourette's tics, it's not just involuntary, it's also sort of explosive and compulsive, you can suppress it (at least some people can), but it tries to explode out of you in a particular way.
As far as what gets called stims, it's not really a fair thing to say any single thing about them.
The reason is that stim is short for self-stimulatory behavior, and self-stimulatory behavior was a term that non-disabled people came up with to explain any number of potentially unrelated behaviors among autistic people.
That is why it can mean...
...becoming greatly fascinated with a sensory experience that most people don't (possibly repetitive, possibly just continuous).
...mannerisms that happen to be fairly unusual among non-autistic people, but that are not consciously embarked upon by autistic people. The autistic person may be able to control it with effort, but if they stop thinking about it it'll happen again, possibly without them even noticing.
...unusual mannerisms deliberately embarked upon by autistic people.
...undoubtedly it's always been used(/misused?) on tics as well.
...etc.
There's other things too.
And of those, they can be for different reasons, from dealing with overload, to an overflow of emotion, to entertaining oneself, to dealing with a lack of sensory stimulation, to... huge amounts of things. Whether they are deliberate or not. (Many times the non-deliberate ones are serving a purpose that the autistic person is simply unaware of consciously, but their brain knows what it's doing.)
So I always find the "Is this a stim?" question kind of vaguely irritating (not the people, just the question), because it makes it sound like the word has a specific meaning.
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"In my world it's a place of patterns and feel. In my world it's a haven for what is real. It's my world, nobody can steal it, but people like me, we live in the shadows." -Donna Williams