As said in the “How to connect Philips Hue devices” page, the SmartThings Hub will automatically detect and add the hue bridge and hue bulbs into SmartThings system.
So, after that, I guess there is a device handler associated with each hue device that runs either on the SmartThings Hub or in the SmartThings Cloud ( most likely in the SmartThings Cloud).
Then, I found the source codes of such device handlers, such as hue-bulb and hue-bridge.
Apparently, such devices are child devices of some other device or SmartAPP, since they are calling their parents using:
log.trace parent.on(this) in the hue-bulb
def result = parent.parse(this, description) in the hue-bridge.
Why the hue devices are added as child devices?
Who is/are their parent(s)?
Where can we get the source code of the parent(s)?