[RichardJ]

This is the same wiring that Vettvet posted earlier, but with most of the non essential circuits removed for clarity.

All copper wire has resistance. The smaller the wire diameter, the greater the resistance. The longer the wire the greater the resistance.
The copper wire between the battery junction block and the Junction below the alternator has a specific resistance because of its size and length. This section of the 12R wire serves two purposes for the price of one.
This wire segment serves double duty as the shunt for the meter movement.
The 18B fused wire and 18B/W fused wire carry a small amount of current to the Ammeter gauge.
If the battery is pushing more current across the shunt than the gen is producing the gauge shows discharge. If the gen is pushing current across the shunt to the battery, the gauge says charging. When the gauge says Zero Amps, the battery is fully charged and the gen is putting out only what is needed above that.