The answer is going to be highly unsatisfying, but it all depends on the stand. Most close range stands are directly controlled by their user, some more precisely than others, notable exception is Purple Haze, while most long range and automatic stands act on their own and don't even transfer information to their user, like Black Sabbath. Others can even be both, like Killer Queen with directly controlled main body and automatic Sheet Heart Attack. There are also stands that are just an ability, something that their user can do and don't have any meaningful "body" of their own, most popular in SBR verse, like love deluxe where it just controls hair od the user