Unity3d ScrollRect Auto-Scroll, Dropdown Use: Places this component in the Template of Dropdown (with the ScrollRect component)

ScrollRectAutoScroll.cs

using System.Collections.Generic;
using UnityEngine;
using UnityEngine.EventSystems;
using UnityEngine.UI;
[RequireComponent(typeof(ScrollRect))]
[AddComponentMenu("UI/ScrollRect Auto-Scroll")]
public class ScrollRectAutoScroll : MonoBehaviour, IPointerEnterHandler, IPointerExitHandler
{
    public float scrollSpeed = 10f;
    private bool mouseOver = false;
    [Header("Navigation")]
    public Selectable selectOnTop;
    public Selectable selectOnBottom;

    private Selectable[] m_SelectableList;
    private Navigation[] m_NavigationList;

    private List<Selectable> m_Selectables = new List<Selectable>();
    private ScrollRect m_ScrollRect;

    private Vector2 m_NextScrollPosition = Vector2.up;
    public void Refresh()
    {
        if (m_ScrollRect)
        {
            m_ScrollRect.content.GetComponentsInChildren(m_Selectables);
        }
        RemakeNavigation();
    }
    void OnEnable()
    {
        Refresh();
    }
    void Awake()
    {
        m_ScrollRect = GetComponent<ScrollRect>();
    }
    void Start()
    {
        Refresh();
        ScrollToSelected(true);
    }
    void Update()
    {
        // Scroll via input.
        InputScroll();
        if (!mouseOver)
        {
            // Lerp scrolling code.
            m_ScrollRect.normalizedPosition = Vector2.Lerp(m_ScrollRect.normalizedPosition, m_NextScrollPosition, scrollSpeed * Time.deltaTime);
        }
        else
        {
            m_NextScrollPosition = m_ScrollRect.normalizedPosition;
        }
    }
    void InputScroll()
    {
        if (m_Selectables.Count > 0)
        {
            if (Input.GetButtonDown("Horizontal") || Input.GetButtonDown("Vertical") || Input.GetButton("Horizontal") || Input.GetButton("Vertical"))
            {
                ScrollToSelected(false);
            }
        }
    }
    void ScrollToSelected(bool quickScroll)
    {
        int selectedIndex = -1;
        Selectable selectedElement = EventSystem.current.currentSelectedGameObject ? EventSystem.current.currentSelectedGameObject.GetComponent<Selectable>() : null;

        if (selectedElement)
        {
            selectedIndex = m_Selectables.IndexOf(selectedElement);
        }
        if (selectedIndex > -1)
        {
            if (quickScroll)
            {
                m_ScrollRect.normalizedPosition = new Vector2(0, 1 - (selectedIndex / ((float)m_Selectables.Count - 1)));
                m_NextScrollPosition = m_ScrollRect.normalizedPosition;
            }
            else
            {
                m_NextScrollPosition = new Vector2(0, 1 - (selectedIndex / ((float)m_Selectables.Count - 1)));
            }
        }
    }
    public void OnPointerEnter(PointerEventData eventData)
    {
        mouseOver = true;
    }
    public void OnPointerExit(PointerEventData eventData)
    {
        mouseOver = false;
        ScrollToSelected(false);
    }
    void RemakeNavigation()
    {
        m_SelectableList = m_ScrollRect.content.GetComponentsInChildren<Selectable>();
        m_NavigationList = new Navigation[m_SelectableList.Length];

        for (int i = 0; i < m_SelectableList.Length; i++)
        {
            m_NavigationList[0] = m_SelectableList[0].navigation;
        }
        if (m_SelectableList.Length > 1)
        {
            for (int i = 0; i < m_NavigationList.Length; i++)
            {
                // First item navigation.
                if (i == 0)
                {
                    m_NavigationList[0].mode = Navigation.Mode.Explicit;
                    if (selectOnTop)
                    {
                        m_NavigationList[0].selectOnUp = selectOnTop;
                    }
                    m_NavigationList[0].selectOnDown = m_SelectableList[1];
                    m_SelectableList[0].navigation = m_NavigationList[0];
                }
                else
                {
                    m_NavigationList[i].mode = Navigation.Mode.Explicit;
                    if (i < m_NavigationList.Length - 1)
                    {
                        m_NavigationList[i].selectOnUp = m_SelectableList[i - 1];
                        m_NavigationList[i].selectOnDown = m_SelectableList[i + 1];
                    }
                    // Last item navigation.
                    else
                    {
                        m_NavigationList[i].selectOnUp = m_SelectableList[i - 1];
                    }
                    m_SelectableList[i].navigation = m_NavigationList[i];
                }
            }
        }
        else
        {
            m_NavigationList[0].mode = Navigation.Mode.Explicit;
            if (selectOnTop)
            {
                m_NavigationList[0].selectOnUp = selectOnTop;
            }
            m_SelectableList[0].navigation = m_NavigationList[0];
        }
    }
}

Update adding the option to generate vertical navigation

Line 18, I don't get how the list gets its objects