That's kind of true, but not necessarily. The staff continues up between the lower and the upper staff, albeit invisibly, so there is no actual break between the two, the lower they are, the lower they are on the keyboard, the higher they are, the higher they are on the keyboard. You can, and do sometimes, cross hands. When listening to Bach, because it's a much older style of music, both hands are playing different "voices." Imagine two singers singing their own part together at the same time.