Probability of Power Source

[9valkyrie]

What is the primary source of power for an iPhone, a MacBook Air, and a MacBook Pro when plugged into the charger?

Hypothetical situation:

If I have a major system update and my battery is at 1% and the device is plugged into the charger, what's the probability of the system update failing or the device being bricked? Is it different for iPhones than notebooks? Is there a device that changes primary power depending on if it's plugged into the charger or not?

 

[now i see it]

The all run off the battery. The charger charges the battery. The charger can supply peak current necessary for some scenarios. Don’t start anything important with an almost dead battery.