I've asked many questions to Kapa.ai but still don't quite get it: Where/how do you set up a trigger for an A/B test? The page where the feature flag is present will be the trigger?

There are a few variables here, but you're right. For tests with feature flags, the experiment runs when the flag is active. For example, if you have a

headline

flag that you use to run an experiment to test different headlines, the tracking callback will only fire when that flag is detected/used.