Function extract_resource 

pub fn extract_resource<R, L, F>(
    commands: Commands<'_, '_>,
    main_resource: Extract<'_, '_, Option<Res<'_, <R as ExtractResource<L, F>>::Source>>>,
    target_resource: Option<ResMut<'_, R>>,
)
where
    R: ExtractResource<L, F, Mutability = Mutable>,
    L: AppLabel,

This system extracts the resource of the corresponding Resource type