Save.cs

public static void SaveJson<T>(List<T> data, string directory)
{
    if (!Directory.Exists(directory))
    {
        Directory.CreateDirectory(directory);
    }

    var fileName = $"{typeof(T).Name}s.json";
    var filePath = Path.Combine(directory, fileName);
    var json = JsonConvert.SerializeObject(data);
    File.WriteAllText(filePath, json);
}