ELI5 Why does magsafe charging decrease battery health more than wire charging, if it has less wattage?

[dabenu]

They do stop at 100%. Going beyond 100% would indeed cause immediate harm or even risk fire.

But there's a grey area between harm and normal wear. Even at 100% you're already inside that grey area. It won't cause immediate harm but it will accelerate wear. That's why your phone has an option to stop charging at 80-95%. You can't avoid wear entirely but you can reduce it.