If you want to have a better feel for how cubic beziers work, I recommend checking outthis desmos link. Play around with the control points and check how the animation changes through time. (Note that the animation in the link is represented by the black line.) MeetSmashing Workshopsonfron...
because some graphs that have the “many to one” situation aren’t necessarily going to be functions; There may be other places (i.e. a couple of other coordinate points) that connect vertically, therefore disqualifying it as a function. These “rules” can also bedifficult to remember...