Suppose you are at a coffee shop and your phone battery is low. You decide to plug it in to your laptop via USB to charge it. At this point the laptop's battery serves as our energy source for charging the phone.
But then you realize your laptop's battery is also low, so you plug your laptop in using the AC adapter it came with. So the coffee shop's electricity is charging the laptop battery.
Now, do most charging circuits in laptops switch to source power (specifically on USB ports) from the AC adapter or not? Would the current draw from the AC adapter increase as a result of having the phone being plugged in, or would it just take longer to charge the laptop battery to full capacity?