Read again I was editing my post when you answered
For declaring it, keep it global since you want to use it both in setup and the loop. You create an instance in the setup, only once, and then That’s the one you use in the loop.
Seems you are not fully getting what’s happening with classes and instances =>You have examples you could study in the library
The & in the signature means they pass by reference. It might mean the data is not duplicated and thus needs to refer to some static data in your app so that the reference stays legit. (I’ve not explored this with the ESP32)