Where do you place code that is not related to any game object directly?

Where do you place code that is not related to any game object directly?

For example I need a c# class SelectionManager and I want it to appear only in my c# project but I don’t want to have it visible as a script in Unity.

Thanks

I believe all scripts appear in the project browser but only classes derived from MonoBehaviour appear in the component menu or are able to be added to GameObjects. For a manager type object I usually just follow the singleton pattern or use a static class.