Automatic quality level dropdown.

QualityDropdown.cs

using UnityEngine;
using UnityEngine.UI;
[RequireComponent(typeof(Dropdown))]
public class QualityDropdown : MonoBehaviour
{
    private Dropdown m_Dropdown;
    /// <summary>
    /// Start is called on the frame when a script is enabled just before
    /// any of the Update methods is called the first time.
    /// </summary>
    void Start()
    {
        m_Dropdown = GetComponent<Dropdown>();

        m_Dropdown.options.Clear();
        for (int i = 0; i < QualitySettings.names.Length; i++)
        {
            m_Dropdown.options.Add(new Dropdown.OptionData(QualitySettings.names[i]));
            m_Dropdown.onValueChanged.AddListener(QualitySettings.SetQualityLevel);
        }
        // Set current quality level.
        m_Dropdown.value = QualitySettings.GetQualityLevel();
    }
}

You need add listener once only.
You can also reduce line count with this:

[RequireComponent(typeof(TMP_Dropdown))]
public class SettingController : MonoBehaviour
{
    private TMP_Dropdown m_Dropdown;
    
    void Start()
    {
        m_Dropdown = GetComponent<TMP_Dropdown>();
        
        m_Dropdown.AddOptions(QualitySettings.names.ToList());
        m_Dropdown.onValueChanged.AddListener(QualitySettings.SetQualityLevel);
        m_Dropdown.value = QualitySettings.GetQualityLevel();
    }
}