macbook adapter for i phone

The Mac adapters can output a variety of voltages one of which is 5volts for those devices that require that.

In other words, yes, you an use the Mac charger to charge the iPhone and no harm will come to it or its battery.

You can use any watt level for your device as long as the connector fit and it does not output more than 5volt

It depend on the charger a 6000watt charger can output 5volt but could also output 380volt