Grossly, B2B units pull down the voltage of the source battery to no lower than 13v. This causes the alternator to ramp up its amperage, typically to maximum output.
Then the B2B pulls up to its rated output from the source battery and then boosts (and now, sometimes, cuts) the voltage to the requirements of the target battery.
Most B2B's have a variety of preset profiles and some, specifically Sterling, allow you to play to your heart's content. (Generally, with LiFePO4, you want to keep the voltage down to around 14v and reduce or eliminate the float voltage. )