'FindPropertyRelative' and an instantiated class inside an instantiated class

I have a problem with comprehending custom drawers and 'FindPropertyRelative' method. For starters, I have these 4 classes:

SectionClass.cs

 using UnityEngine;
 using System;
 
 [System.Serializable]
 public class SectionData : System.Object
 {
     [SerializeField]
     public int ObstacleType;
     [SerializeField]
     public Vector3Serializer ObstaclePosition = new Vector3Serializer();
 
     public SectionData()
     {
         this.ObstacleType = 0;
         this.ObstaclePosition.Fill(Vector3.zero);
     }
 
     public SectionData(int ObstacleType, Vector3 ObstaclePosition)
     {
         this.ObstacleType = ObstacleType;
         this.ObstaclePosition.Fill(ObstaclePosition);
     }
 }
 
 [System.Serializable]
 public class Vector3Serializer : System.Object
 {
     [SerializeField]
     public float x;
     [SerializeField]
     public float y;
     [SerializeField]
     public float z;
     
     public void Fill(Vector3 v3)
     {
         this.x = v3.x;
         this.y = v3.y;
         this.z = v3.z;
     }
     
     public Vector3 V3 { get { return new Vector3(x, y, z); } set { Fill(value); } }
     
 }

Test.cs

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 public class Test : CustomFunctions
 {
     public List<SectionData> testList = new List<SectionData>();
 
     void Update ()
     {
         //Do Stuff
     }
 }

SectionCustomEditor.cs

 using UnityEngine;
 using UnityEditor;
 using System.Collections;
 using System.Collections.Generic;
 
 [CustomEditor (typeof(Test))]
 [CanEditMultipleObjects]
 public class SectionCustomEditor : Editor
 {
     SerializedObject obj;
     SerializedProperty testListProp;
 
     void OnEnable ()
     {
         obj = new SerializedObject(target);
         testListProp = obj.FindProperty("testList");
     }
 
     public override void OnInspectorGUI()
     {
         obj.Update();
 
         for (int i = 0; i < testListProp.arraySize; i++)
         {
             SerializedProperty section = testListProp.GetArrayElementAtIndex(i);
             SerializedProperty otpProp = section.FindPropertyRelative("ObstacleType");
 
             //This bellow part is the problem and always returns null
             SerializedProperty v3Prop = section.FindPropertyRelative("ObstaclePosition");
 
             otpProp.intValue = EditorGUILayout.IntField ("Obstacle type: ", otpProp.intValue);
         }
     }
 }

The problem is I can't understand how to access the whole 'Vector3Serializer' class and it's fields 'x', 'y', 'z'.