Battery State of Charge (Battery SoC) is calculated as an ongoing process by measuring the voltage and the current used. These two are multiplied to determine how many Watts are used. The wattage at various times (maybe every second) is then added to predict how long the battery will last at the current rate, which clearly changes.
This ‘rate’ is then used with other ‘data/rates’ to determine the approximate life cycle of the battery. This in turn will impact the charging rate as a battery that is depleted in terms of cycle life will be charged more carefully/slowly/less current. Hence the AI or algorithm that is used to charge the battery. Hence the ability to calibrate the AI for a battery as the data held may not be efficient or optimal.
On the voltage side alone. The phone uses Quallcom Q3 AI to communicate with the charger to decide what voltage can be used given a) the state of the battery (cycle degredation) or SoB and b) the SoC
So charging is not based on battery voltage but SoC
For example if a battery is under heavy load for a while then the voltage will be low but that does not mean the battery has a low SoC
So the AI has to work out what current, from Wattage used, the phone should be charged at.
The Q3 AI will, if connected to a Q£ compatible charger then decide what voltage to apply to the phone.
If the AI doesn’t communicate between phone and charger the default may well be 500mA at 5V ~ ie old PC USB so as not to charge the phone at a too high a rate.
The AI knows what is the best rate but if that cannot be communicated to the charger is does the default low, irrespective of the battery voltage.
A bit off topic and probably not quite how it works
See also
UPDATE:
Just to sumarise.
The voltage of a battery drops immediately when a load is connected. The voltage will recover when the load is removed.
At the two moments just before removing the load and after the SoC charge is the same but the voltage isn’t. The AI allows the phone to be charged under any laod as it calculates the SoC and does not react to the voltage.