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C# words:

var flat = list.SelectMany(l=>l).ToList();

Java words:

List<model> flat = list.stream().flatMap(List::stream).collect(Collectors.toList());

list.stream().flatMap(model-> model.stream()).forEach(System.out::println);

The data structure dictates, let’s loop

        public static IEnumerable<T> GetItems<T>(this List<List<T>> list)
        {
            foreach (var child in list)
            {
                foreach (var item in child)
                {
                    yield return item;
                }
            }
        }

        public static IEnumerable<T> GetNestItems<T>(this System.Collections.IList list)
        {
            Type type = null;

            foreach (var item in list)
            {
                if (type == null) type = item.GetType();

                if (type == typeof(T))
                {
                    yield return (T)item;
                }
                else if (type.GetGenericTypeDefinition() == typeof(List<>))
                {
                    var items = GetNestItems<T>((System.Collections.IList)item);

                    foreach (var t in items)
                    {
                        yield return t;
                    }
                }
            }
        }

Don’t cycle yourself. Or catch other functions to help you complete the loop.