cpldcpu did a teardown of an external USB battery:
The device has a USB micro-b socket which is used as 5V input for charging, and a normal USB-A socket as 5V output. The output power can be turned off and on by a toggle button. There are LEDs to indicate active power out (blue) and charging (red) states.
The pictures above show the innards of the device. Most space is taken up by an ICR18650 LiIon battery, which are relatively common devices with 2600mAH. In addition, there is a tiny tiny PCB. The rear side of the PCB is dominated by a 4.7µH inductor, which is part of the boost converter to convert the 3.7V of the battery to the 5V USB output.