If that voltage needs to be DC, it is rectified through a few diodes, and maybe regulated ... in the lower leg of the voltage divider — the same circuit upside-down. The fuse and safety ...

The main circuit is powered by your 13.8 V ... Zener diode pulled from an old scrapped TV. The diode is referenced to a voltage divider, and voila! Your LM7805 is now putting out a steady 9 ...

The circuit of a voltage divider may be drawn with the two resistors vertical, not horizontal. If there are two resistors in series across a voltage source, then the circuit is a voltage divider.