Difference between JSON when serializing (list<T>) and serializing (.NET string)

string str= "[{ \"firstName\":\"Todd\",\"lastName\":\"Grover\",\"gender\":\"Male\",\"salary\":50000},{ \"firstName\":\"Sara\",\"lastName\":\"Baker\",\"gender\":\"Female\",\"salary\":40000}]"; 
    Console.WriteLine(str); 
    var js = JsonConvert.SerializeObject(str); 
    Console.WriteLine(js);

------------------------------------------------------------------------

    

Employee
--------

    public class Employee
        {
            public string firstName { get; set; }
            public string lastName { get; set; }
            public string gender { get; set; }
            public int salary { get; set; }
        }

Program.cs
----------
    var employee1 = new Employee
                {
                    firstName = "Todd",
                    lastName = "Grover",
                    gender = "Male",
                    salary = 50000
                };
    
                var employee2 = new Employee
                {
                    firstName = "Sara",
                    lastName = "Baker",
                    gender = "Female",
                    salary = 40000
                };
    
                var listEmployee = new List<employee>();
                listEmployee.Add(employee1);
                listEmployee.Add(employee2);
                var js = JsonConvert.SerializeObject(listEmployee);
                Console.WriteLine(js);