What is the difference between I have seen people do it and I have seen people doing it?
The question focuses on "do" vs. "doing", but the issue is more complicated than that; it depends on the context.
In the gerund form, "I have seen people doing it", the gerund describes an uncompleted or in-process action. So this sentence implies that you actually witnessed the action.
The complication comes with "do", "I have seen people do it", which may or may not mean that you witnessed the action.
The sentence refers to past activity, so this usage of "do" is similar to using "did" -- it's a reference to the fact that it happened rather than a description specifically of witnessing the process of it happening.
Whether or not it's clear if the action was actually witnessed depends on the other verb, and whether that one is ambiguous. For example, "observe" means to watch (M-W), so if the sentence was "I have observed people do it", that would unambiguously mean you saw the action (it would probably be used with "doing", but that's a different issue).
Your example uses "seen" as the other verb. "Seen" has a lot of possible meanings. It can mean perceived by the eye, in which case you would have witnessed the action. It can also mean to come to know, which doesn't necessarily mean seeing it happen (see M-W). So "seen" doesn't distinguish the intended meaning.
If the sentence included a specific person, time, or place, that would imply the "witnessed" meaning of "seen". This sentence doesn't, just a generic reference to "people". So nothing else in the sentence clarifies the precise intended meaning (did the person witness the action or not?).
The use of "do" sets the stage for multiple potential meanings, and then it is a matter of the context provided by the rest of the text as to whether "do" vs. "doing" might have different intended meanings.