Not looked at the file but…
A Symbol needs to be duplicated to work as an indivdual instance.
If you just copy a symbol and edit any one of the copies, All copies will change. The best way to look at them when editing and setting up is they are legion…, ok bad joke. They are all ponters to the same object but can be displayed as many.
Duplicating one will break off a version which can be edited.
The custom behaviour remains where you set it up and can be changed there, This will be on the Symbol Inspector in this case.
Once you create a CB it becomes avalible in the action Inspectors or Keyframes. You cannot edit there.
They do take a bit to get you head around
Look at custom behaviours as a worm holes to a symbol or scene.
It bypasses all the normal roots to symbol or scene allows you to tunnel directly to the symbol/scene and get it run actions ( set in the CB )
You first setup a custom behaviour in the Symbol or Scene.
And then from amother scenes link to it via selecting it in the actions.
It is best you look at Symbols in the Docs.