The built in hub is a complete smartthings hub. It just has limited abilities to send info back to the ADT side. Also all ADT dual branded sensors are available in the Smartthings side to setup automations
Thanks for the complete answer.
If I have SmartThings hub controlling the existing sensors (lighting, buttons) and want to add ADT Hub to do just the security (contact, motion and presence sensors) - can the two hubs/systems co-exist?
The benefit I see in it is that I isolate the security components to ADT hub and have SmartThings hub do home automation tasks. I would not try to add home automation stuff to ADT Hub or vice versa.
Yes; but remember that the ADT Security features only work with ADT sensors, so if you’re aiming in that directly, you might as well skip ADT entirely and get one of the new and more competitive alarm systems like Ring or Nest.