Overcharging the phone is the no 1 cause of battery degradation.

Overcharging above 85% is considered to be non beneficial to lithium ion batteries. The characteristics of lithium ion batteries would be that it doesn't prefer below 20% battery level and over 85% battery level. So charging it over this point would be considered overcharging it.

No, your phone doesn't really stop charging when it reaches 100% battery level. The phone would trickle charge the phone from 80% onwards till it reaches 100%, stop the charge and then continue to top up to 100% once the battery levels drop, if your phone is plugged in.

If you want to do that, you could get charging cables which would stop charging once it reaches 100%. McDODO cables are one of the brands which has produces cables which has such features.