All the logic of the handles come together in the AdvancedHandles class. Here you find all methods for each handle. When you call a handle method, it draws the handle in the scene and calls different methods if needed, like all interactions. The interactions are inside the class InteractableHandles so we don't have to copy them for each handle.
The meshes are stored the MeshTemplates class. The data is stored in arrays. The AdvancedHandles class can get the meshes from this class. The first time they are created from those arrays in a private static variable. After that, they give this static private variable when a class ask for it for performance.