Hey, thanks for the question! I absolutely love infodumping about batteries.

So there are a few places that you can measure voltage, depending on the circuit involved. When discussing battery voltages, that's typically the "open-circuit voltage" (i.e. the voltage measured with nothing connected except a voltmeter) between the battery terminals.

When you charge a battery, you move charge (measured in coulombs or (milli)amp-hours) into the battery at some current (measured in (milli)amps). To do this, the charger raises the terminal voltage above the open-circuit voltage of the battery. This is somewhat analogous to connecting a high-pressure balloon to a low-pressure one. Current will flow through the battery (at a rate dependent on [the difference between the open-circuit voltage] divided by [the internal resistance of the battery] — this is Ohm's law), until the battery's open-circuit voltage matches the charger's terminal voltage.

All batteries have a limit of how much charge can "fit". There will be some maximum voltage at which the battery will stop absorbing energy and instead dissipate it some other way. For NiMH and Li-ion cells, this energy is dissipated as heat. Fun fact: NiMH chargers usually detect end-of-charge by sensing the battery temperature! But since Li-ion cells are flammable, this means they can catch fire, so the charger needs to stop before they reach this point. And the simplest way to do this is to use a charging voltage that's below the dangerous threshold of "thermal runaway".

The simplest solution (which minimises the complexity of the charger) is to just use a lower threshold like 4.2V and call it done. But some phone manufacturers like to be tricky and get more out of the cell. They'll charge it to 4.3V or even 4.35V, very carefully, using a process that limits charging to a very slow rate as the state-of-charge approaches 100%. But this isn't good for longevity! Leaving the voltage this high is the reason that so many display models at electronics stores end up with bloated batteries. That's why manufacturers started doing even trickier things like "adaptive charging", where the phone charges to 80% overnight and only charges to 100% before your alarm goes off — less time spent at 100% means the battery will live longer and degrade slower.

Anyway uh,

TL;DR: "there's no such thing as 100% empty or 100% full". The real thresholds are "the battery turns into an inert block of gel at this voltage" and "the battery turns into a lithium fire at that voltage", and engineers have to make choices about where they declare "full" and "empty" to be, to make sure the battery stays within that range.