Note2: if it's just a generic script that doesn't need to be attached to a specific node (like audio node, sprite node, etc.) You can add a generic Node as a child of the root node and use it just to attach your script. I use it for my Finite State Machine for example

Note3: when working with nodes, it's important to pay attention to the node hierarchy. It's simple when a node needs to communicate with other node in the same scene, but it can turn a bit complicated when doing a node in one scene component to communicate with a node in other scene component