I understand that a device only draws as much current as it needs. So assuming the power supply and the device have the same voltage (5V), using a power supply able to provide more amperage (= more watts) will not change anything.
However if the power supply can't provide enough current (nominal current of the device), that will either overheat the supply and/or the voltage in the circuit will drop.
Also regarding the voltage it is important that it be the same.
If that of the power supply is higher than the device, the current in the circuit will be too high (assuming the power supply can supply this new level of amperage) with a risk of frying the device.
if that of the power supply is lower, then the device might not be working properly as not enough current circulates inside.
My question is regarding the statement that new iphones can be charged with both the 5W and 12W charger, the charging time being faster with the latter.
How comes? To me either the 5W matches the device's inputs and the 12W shouldn't change anything, that is, it works but not faster. Or the 12W is the correct one but then it would mean that with the 5W the power provided is too low and we are in the case explained at the top where the power supply rating is below the device requirements, which can create issues.
The only explanation I can think of is that the circuit in iphone is more complex and somewhat intelligent with variable internal resistances to regulate the amperage and adapt to the power supply specs. Would that be correct?