You need a HomeKit compatible app, and hardware. I've used the app "Home - Smart Home Automation" by Matthias Hochgatterer. From the Home app you create and name a HomeKit Home, then you can create rooms and add your individual accessories to each room (so you can tell Siri to adjust a whole room at a time). You can also create scenes.
I've got a "good morning" and "good night" scene which turns on at a dim setting, and turns off respectively, all lights in my house. I can also say "Hey Siri, set the living room lights to 20%" or "Hey Siri, set the living room lights to pink" etc.