Hi,
Close to the input jack there's a small pream that provides two different levels to the preamp/effect section. The preamp/effect section is the main board that sits in the middle with rather big (2"x2" or so chip(s) on it). The signal from the input jack is first transformed to digital and from there it goes to the big chip (a Ddigital Signal Pprocessor) where effects, patch management, volume and tone settings take place. The DSP is controlled by a set of pots/knobs all gathered on the front Printed Circuit Board; electrical values/signals are read from the positions of the knobs to "tell" the DSP how to manage the guitar signal. Finally the signal is turned back into analog again before entering the Main Amplifier section, another PCB with essentially one big chip on it, itself attached to a heatsink. The big transformer is to change the mains voltage to something adequate to the power amplifier and other preamp sections.
Hope this helps,
-Patrick