This
Generally MOST phones when charging work as a pass through, when your using it while plugged in the power is more or less comming from the source rather then the battery
Regardless it would not matter, modern electronic batteries are lithium and not Nickel Cadmium like they used to, they do not build up memories or need to be “trained”
Although it is not a good thing to allow your cell phone or any Lithium based batteries to completely discharge as it will shorten the overall life span of the battery down the road