A monster that is in optimal range to attack its focus (i.e. at melee range if they’re not ranged, within their range but not immediately adjacent if they are ranged) doesn’t generally move, no. If they have multiple targets on their attack, either by default or on that particular action card, they will move to use as many of those additional targets as they can and still attack their focus, though. (I think).
It’s one of the most complex elements of the game, so don’t feel too bad about not getting it right away. I’m dozens of scenarios in and I’m not sure I’m right about every possible edge case we’ve run into.