[Serialize] Attribute with JSON

I need some clarification on using Json.Utility.ToJson. In the documentation it says that plain classes must include the [Serializable] attribute. However take the following example:

     using System;
     using System.Collections.Generic;
     using UnityEngine;
      
     public class JsonTest : MonoBehaviour
     {
      
         void Start()
         {
             string json = SerializeToJson();
             DeserializeFromJson(json);
         }
      
         string SerializeToJson()
         {
             NonSerializedClass instance = new NonSerializedClass();
             string json = JsonUtility.ToJson(instance);
             Debug.Log(json); // Result: {"valueOne":10,"valueTwo":20}
      
             return json;
         }
      
         void DeserializeFromJson(string json)
         {
             NonSerializedClass instance = JsonUtility.FromJson<NonSerializedClass>(json);
             Debug.Log(instance.valueOne); // Result: 10
         }
     }
 
     public class NonSerializedClass
     {
         public int valueOne = 10;
         public int valueTwo = 20;
     }

In this example, the NonSerializedClass (as the name implies) does not use the [Serializable] attribute, yet it still gets serialized correctly, and deserialized correctly. I would expect this to return empty JSON as a result of not including the attribute.

However, if I try to place the class in a List<> then I get the results I expect, take the following example (NOTE: I still don't include the attribute on the NonSerializedClass):

     using System;
     using System.Collections.Generic;
     using UnityEngine;
      
     public class JsonTest : MonoBehaviour
     {
      
         void Start()
         {
             string json = SerializeToJson();
             DeserializeFromJson(json);
         }
      
         string SerializeToJson()
         {
             List<NonSerializedClass> dataList = new List<NonSerializedClass>();
            
             dataList.Add(new NonSerializedClass();
             dataList.Add(new NonSerializedClass();
             dataList.Add(new NonSerializedClass();
      
             NSList container = new NSList(dataList);
      
             string json = JsonUtility.ToJson(container);
             Debug.Log(json); // Result: {}
      
             return json;
         }
      
         void DeserializeFromJson(string json)
         {
             NSList container = JsonUtility.FromJson<NSList>(json);
             Debug.Log(container[0].valueOne); // Null reference
         }
     }
 

     public class NonSerializedClass
     {
         public int valueOne = 10;
         public int valueTwo = 20;
     }

     // Wrapper
     public class NSList
     {
         public List<NonSerializedClass> dataList;
      
         public NSList(List<NonSerializedClass> _dataList)
         {
             dataList = _dataList;
         }
     }
 

This behaves as expected, and returns an empty JSON and a Null Reference in the deserialization. This can be rectified by including the [Serializable] attribute. But this then begs the following question, how comes the Wrapper class does not need to include the Attribute in this circumstance?

I would like to know the following:

• Why does my NonSerialized class correctly serialize to JSON without the [Serializable] attribute?

• When adding the class to a List<> using a wrapper class to serialize the List<>, how comes only the class in the collection needs the attribute, and not the wrapper class?