It is pretty simple, in concept. One glob of transistors
constitute the "pull up" system (that provides a current
path from the positive supply rail to the load) and another
glob of transistors constitute the "pull down" system (that
provides a current path from the load to the negative supply
rail). If both these systems are partly on, when no load
current is needed, this produces a current path from the
positive supply rail to the negative rail, draining power
from the supply, continuously. This is called class AB.
But it allows the amplifier to switch from pull up to pull
down, very smoothly, by just increasing the pull down
current while simultaneously reducing the pull up current
(or vice versa). The LM324 was optimized for very low
drain, battery operation, so instead of class AB, they set
it up so that, at idle, neither pull up or pull down is in
operation, so the output stage draws no current. But it
takes a little time to get the pull up or pull down system
operating, once either is needed. This dead time represents
a chunk of the output waveform that is malformed, and is
called cross over distortion (when the output current
crosses over from positive to negative, or vice versa).
Yes. It simplifies lots of aspects of most circuits. Once
you understand these fewer aspects, then you add the details
that let the system operate from one supply.
I am talking about your education, not your final circuit.