iphone 15 battery optimization

With iPhone 15 you can set the battery to stop charging when it gets to “about 80%”. For years it was stated the battery won’t get damaged if left charging when it gets to 100% as the phone will stop the charging process. So if this is true I’m curious as to why a lot of iPhone 14 owners have lost a lot of their battery life.

so my question is this, and I do realize this is more for an engineer- with the 80% charge setting does the phone somehow stop the charging current from reaching the battery or does the current reach the battery and the battery just stops charging? If the latter than how is this any different from leaving phone plugged in at 100%?