It boils down to applying a carefully controlled charging voltage (higher than the current battery voltage) to the battery. I would assume that the voltage is adjusted to achieve a specific charging current.

With PPS (programmable power supply), AFAIK the phone will typically ask the charger for a voltage twice as high as it wants to send to the battery, possibly a bit more to compensate for losses in the cable, then halve that with a highly efficient charge pump (https://en.wikipedia.org/wiki/Charge_pump).

The best (most "gentle") way of charging a battery would likely be a phone that is intentionally not trying to fast charge, connected to a PPS-capable power supply. This would minimize losses and thus heat within the phone without charging the battery faster than necessary. I suspect that the difference to charging off a non-PPS charger is negligible in practice.

Wireless charging creates a lot of waste heat, which isn't great for the battery.

All charging does is reverse the flow of lithium ions through the electrolyte that happens when the battery discharges.

Wireless charging is usually worse long term because of the extra heat the coils create from the waste energy. It's probably about as bad as the really fast charging phones do now though. I principally have mine do a slow charge over night these days though as is mostly last through the day.

It's basically as simple as a buck converter that reduces the incoming voltage.

Phone batteries need either 4.2V or 4.35V to charge fully, so whatever voltage comes in (5-20V typically) at whatever current, gets reduced down to the correct voltage and current to charge the battery.