Every laptop has an internal power supply to produce the internal voltages needed by the machine. For example, the 1.5 VDC (or whatever) to power the CPU. All the "brick" does is charge the battery and supply power to the internal power supply while the battery is charging or charged. Think of it this way:
The internal power supply powers the CPU, GPU, display and other circuitry.
It receives "raw" (different voltage) power from the battery and/or the external "brick".
The power supply decides where to draw power from and charges the battery when it can (a little like a hybrid car does).
The power supply creates the reset signal needed to initialize all the digital circuitry on startup.
The power supply monitors operations and (usually) shuts down the machine in an orderly fashion if the battery is getting too low on charge with no external power.