Does it go without saying that whenever the 1AC is working the camera he is both aiming it AND focusing it, or is there some kind of system whereby one person can aim the camera and a different person focus it?
It's essentially a two person job...a Camera Operator and a 1st AC (Focus Puller in old terminology).
The camera operator is responsible for physically operating the camera and maintaining composition and camera angles throughout a given scene or shot.
The operator points the camera as indicated and desired by the director and the 1st AC maintains the focus despite changing angles and close-ups.
A focus puller, or 1st assistant camera, is a member of a film crew's camera department whose primary responsibility is to maintain image sharpness on whatever subject or action is being filmed.
"Pulling focus" or "rack focusing" refers to the act of changing the lens's focus distance setting in correspondence to a moving subject's physical distance from the focal plane. For example, if an actor moves from 8m to 3m away from the focal plane within a shot, the focus puller will change the distance setting on the lens during the take in precise relation to the changing position of the actor. Additionally, the focus puller may shift focus from one subject to another within the frame, as dictated by the specific requirements of the shot.
The Focus Puller is unlikely to replaced by technology soon but new technologies and devices are likely to make the job somewhat easier.
Traditionally, the focus puller used only his marks and his own well-developed sense of distance estimation to achieve good results. Over the last decade, the increased use of digital cameras, higher-resolution video taps and/or on-camera monitors have provided focus pullers with additional tools to help maintain proper focus. A high-definition monitor can be particularly useful when a fast-paced production simply does not allow time for the focus puller to set and check all marks that may be needed, or if no rehearsal will be provided.
@Tetsujin notes from the comments
...the poor focus-puller doesn't have to run round hanging off the end of the lens any more, he can use one of these & remote monitor the focus