Yes you would set it up in the app you use to configure HomeKit. Unfortunately Apple doesn't have a product t one weirdly enough there is an app called Home I use that looks as if Apple designed it and let's you configure all aspects of HomeKit
When you setup a trigger in your app, is it your app that watches for the trigger conditions to be true and fires off the action?If you are looking for a free option to check out Triggers our ConnectSense app supports all the HomeKit profiles and supports triggers. You would first need to setup a scene for what you would like the result to be and then you can set up a trigger based of a number of different things. This can be the state of a device (Ex: I turned off this light, now I want this scene to run to turn off all the other lights) or it can be a Timer (Ex: Its 1 AM and I am going run a scene to turn off certain lights).
Let me know if you have any other questions about this, happy to help!
When you setup a trigger in your app, is it your app that watches for the trigger conditions to be true and fires off the action?
i.e. If I set a trigger to run at 7pm to turn on the porch light scene, but then turn off my iPhone completely at 6:50pm, will the trigger still fire at 7pm?