It’s not actually charging.
Your Mac uses the power from the charger for its power needs without going through the battery. If that power is not enough then it takes some more power from the battery. If not all power from the charger is needed and the battery is not full only then will the battery be charged.
No. Apple manages the full charge state of all new hardware since 2017 on average (macs got it earlier and iOS a bit later).
The only thing that damages your MacBook class battery is lots of fast and hot (50 C) charging, or storage above 60 C. I suppose never ever discharging it might lessen the FCC but I can’t imagine you never draining it once every 6 to 9 months.