The alternator with its voltage regulator forms a constant voltage power supply.
The alternator itself, with a constant field current, and a constant resistance load, will generate a voltage roughly proportional to RPM. This is no good, which is why the voltage regulator is added, and this varies the field current so as to keep the output voltage relatively constant.
The regulator is usually rather primitive which is why the resulting voltage is not perfectly constant. However, in my TB20 I am getting 28.something and it varies over a fraction of a volt only.
The regulator doesn't drop the alternator output voltage. The output voltage still comes directly from the alternator output. The regulator controls that voltage by varying the field current, which is much lower than the normal load current.