The voltage of a Lipo corresponds to it's charge level. So a Lipo at 4.2V (or in case of high voltage Lipos 4.35V) is full.

Up to ~80% of the charge, the lipo is charged by current limiting (basically, the voltage of the charging circuit raises so that it stays so much above the cell battery that it's charging at a set current). This is the fast charging part of the charging process.

After the charging voltage reaches the maximum allowed cell voltage (4.2V/4.35V), the charging circuit cannot go above that voltage because it would risk overcharging and blowing up the cell. So the carging circuit holds the voltage at maximum level until the cell voltage catches up. Since the voltage difference shrinks with every bit of charge on the cell, so does the charging speed.

That's why you only see "Charges the phone from 0-80% in X minutes" in the ads, and not 0-100%.

This means, that the charger in incapable of overcharging the phone.

But keeping the charger running even though it doesn't charge the phone anymore wastes energy, so what they do once you reach 100% is that it will disable the charger until the voltage is down to 95%, when it will resume charging. That's why it's quite likely if you unplug the phone after charging overnight, that the battery is not at 100%, but slightly below.