On ThinkPads this can be (or maybe used to be) configured in one of the preinstalled ThinkVantage utilities (Power Manager). But I believe that the whole charging logic itself is really in EC firmware and this application is only user interface for that.

As an aside: the mechanism works in slightly different way (at least on all of my 4 thinkpads of different generations). There are two thresholds, maximum battery capacity when charging can start and capacity when charging stops, the aforementioned application even seems to have some kind of logic to set these thresholds according to battery aging.

Also, trying to keep Li-ion cells charged to constant state of charge is not good for their lifespan. What is commonly known as trickle-charging (charging by infrequent, short but large pulses of current) is actively discouraged by most cell manufacturers, sometimes even citing safety reasons.

It's definitely a separate app. Lenovo has various versions of "Power/Energy management" apps available depending on the specific model of the laptop. Not sure about other manufacturers though.

From experience, it's a separate app but manufacturer specific. Haven't had much luck looking for a generic app.