You are correct almost everything has to be done in the smartthings mobile app. Just be away that everything the app and system can do ARE NOT obvious. There is alot of supported app and utilites hidden from view and not obvious to the new user.
Smart lighting is part of smartthings. If you open your app and select marketplace then smart apps you will find smart lighting and other useful app in there.
You might find this link useful. It has alot of information. Somew of the info and screen shots are behind the updates but still applies.
To use smart lighting to control your scenario. I would make 2 rules one to turn the light on with motion and another to turn the light off when the iris sensor see’s no motion. Just be aware if the iris sensor is not very close to where your sitting it will not see you move if you sit in a chair. So it will turn the lights off on you unless you tinker or find a second input to help keep it active.
Most people get around this using Core as it is very versatile and you can program in a delay before the light turns off where the sensor has time to see you again.
Hope that helps you get going and able to get your lights working. If not ask more questions.