shoppingList selector

        private IEnumerable<string> FormatNonFunctional(List<string> shoppingList)
        {
            var index = 0;
            var result = new List<string>();

            _outp.WriteLine(shoppingList.Count.ToString());

            return shoppingList.Select(item => (index++).ToString() + item.Capitalize());
        }

@arialdomartini, does it solve?

The body of the lambda expression can be replaced by:
item.Capitalize().PrependNumeration(index++)

Even better:

private IEnumerable<string> FormatFunctional(List<string> shoppingList, int startingIndex = 0)
    => shoppingList.Select(item => shoppingList.Select(item => item.Capitalize().PrependNumeration(startingIndex++)).ToList();