Hey, those trigger actors are from Epic’s Content Examples project.
To answer your question: The array is populated in the level, you need to click on a trigger actor in the world outliner and look at the array in the details panel to see which actors it is capable of influencing (via the Call Trigger Actor interface, which any such actors must implement). IsValid is not used for selection. It is just a normal validity check, no magic under the hood.
Suggest checking out Epic’s Content Examples project if you need more information.