First you need to put your list inside the scope of the class like this:

using System.Collections.Generic;
using UnityEngine;

[System.Serializable]
public class Level {

    public List<Level> levelList = new List<Level>(); //List inside the class

    public string Name;
    public Sprite Icon;
    public bool Unlocked;
    public bool Interactable;
}

On your accesor class you need to have a class variable so that you can access that level instance. Then somewhere get the list of that level instance, as in for exmaple in the Start of the code below.

using System.Collections.Generic;
using UnityEngine;

public class ListGetter : MonoBehaviour
{
    public Level level;
    private List<Level> levelList;
    
    void Start()
    {
        levelList = level.levelList; //get level list
    }
}

You would need to define how your Level instance is created, and the class that holds the instance of the class to provide a more accurate way of accessing the levelList of your Level instance, which I think is what you are asking