The Selector component has OnSelectedUsable() and OnDeselectedUsable() UnityEvents and SelectedUsableObject and DeselectedUsableObject C# events for this purpose.
In your PlayController script, you can hook into the C# events. Example:
Thank you for always replying so swiftly. I had looked into using the ProximitySelector's OnSelectedUsable() in the inspector. However, when I drag in my CharacterController and select my PlayerController's class, I don't see an option to access any of the variables in the class
What can I select to modify the PlayerController's IsInRangeOfNPC bool?
UnityEvents generally don't let you change variable values directly. You can write a C# method in your script and connect the UnityEvents to that. Example: