They can rewind alternators to increase their output current. The voltage is controlled by your voltage regulator, which depending on the vehicle may be an external box or internal to the alternator.
There's logic to the way a vehicle charging system is built. There's voltage and temperature characteristics that dictate the ideal charging parameters for a battery of any type. For example most of the time the charging voltage is given as something like 13.6V at 25°C and will go higher or lower in voltage as you change temperature. It might be 14.0V at 0°C or 13.2V at 40°C (these are just examples, the battery manufacturer will specify them).
In that vein what you've heard about going from a flooded lead acid to an AGM is true in theory, they generally require a higher voltage to fully charge. However they also require their max voltage to be more carefully controlled to avoid over-charging. They are sealed so if you go too high and cause the electrolyte to boil and vent there's no way to put it back in so if this happens the loss in capacity is permanent while with a traditional flooded you can just add back in water.
So my point would be to tread carefully with trying to dial in charging with an alternator. The OEM picked the safest profile that won't be a problem for most any battery. They tend to fall into being able to do two of the steps, bulk charge (tons of current) and float (safe long term voltage), which does about 90% of an ideal charge. The third step of a minimally "ideal" profile is absorption is where the ideal voltage put on a battery to top it fully.
The best way IMO to deal with this, particularly with AGM that requires a pretty tight window optimally, is to use a charger at home regularly that's designed to work with your specific battery. If you want to do it on-board the best solution is a DC-DC charger that converts the crudely controlled alternator output to a well controlled multi-step profile.
