Alternator vregs or generator vregs?
Alternator vregs are pretty simple:
Rectified voltage from its own diode is fed into comparator (1-4 phase rectifier is for actual output), if voltage is under setpoint, feed said voltage via transistor or whatever to the rotor electromagnet. If voltage is too big, dont feed the voltage to the rotor coil. Usually its configured so it doesnt oscillate.
If there is no rectified voltage, you can rev the alternator until residual magnetism wakes it up, but usually they have the initial exiting provided by the idiotlight (enough exitement for the vreg to start operating, diode from fields drives this high so idiotlight goes out).
2 connectors for brushes (or 1 if ground is shared)
1 connection for exiter (idiotlight)
1 ground
1 for output/battery connection (depends on setup)
1 for stator/fields connection (usually internal diode, provides power for rotor coil, depends on setup).
Battery power never drives the rotor coil directly as that is just asking for trouble (maybe Lucas....).
I've tested couple with variable power supply (feed voltage into field input and measure when voltage on brushes drops down, I think I had 15ohm load resistor connected to brushes).
Generator vregs are a lot different as permanent magnet generators just spits out stuff depending on speed.
Remember: like DC motors so no rectifiers, no diodes (alteast in the old ones).
This is just example as there are many ways this was done.
They usually have a relay system that feeds from the generator.
When voltage is under battery voltage the relay is open (car not running, no need to connect battery to generator coil to waste power, closed when voltage is good). If voltage is high, throw relay contacts to incude resistor in series. If voltage is dangerously high, disconnect with relay.
Usually 3 connections: ground, generator positive, battery positive (or opposite if system is running positive ground).
May have external load resistors but thats pretty rare as generators are rather weak.
Never tested one as they either work or dont, voltage to generator in should engage relays depending on voltage.
Edit:
Ok had old Lucas 127 alternator regulator hanging around and it only has fields in, gound and one brush connection, the idiotlight (excitement) is connected directly to brush and the light goes out when the rotor has high current (voltage) across it.