Maximum alternator output usually occurs at several thousand (alternator rotor) RPM, and with typical drive ratios alternator RPM is usually two to three times engine RPM, so the alternator will not deliver maximum load at idle, but they are usually sized and set up with drive ratios to handle maximum vehicle system electrical load at idle.

I don't have any data for Bosch alternators, but seventies vintage Delco alternators have a specified maximum rotor speed of 18,000 RPM, and I think they delivered maximum rated amperage at about 5000 RPM, but this was usually a good deal higher than maximum vehicle electrical load in order that the alternator support maximum vehicle electrical load at idle.

The voltage regulator opens and closes the circuit to the rotor windings, which act as the field in order to maintain system voltage in the 13.5-14.5 volt range. The raw output is three-phase AC, which is then converted by the rectifier bridge and diode trio to a DC voltage, and a typical voltmeter will measure the average.

If you look at output on a fast scope it will have a sawtooth appearance.

Duke