Custom Foldout Example

// Stats class
using UnityEngine;

public class Stats : MonoBehaviour
{
    [SerializeField]
    private int health;
}

// Editor Class
using UnityEditor;

// Note the use of CustomEditor.
[CustomEditor(typeof(Stats))]
public class StatsEditor : Editor // Note that we inherit from Editor.
{
    private static bool showGeneralSettings = true;

    public override void OnInspectorGUI()
    {
        // Create the foldout, and link it to our bool.
        showGeneralSettings = EditorGUILayout.Foldout(showGeneralSettings, "General Settings");

        // Create a space to separate this foldout from the other potential variables
        EditorGUILayout.Space();

        // This would make more sense as a switch when you add more settings.
        // If the foldout is opened.
        if (showGeneralSettings)
        {
            DisplayGeneralInfo();
        }
        // Apply changes made in the editor.
        serializedObject.ApplyModifiedProperties();
    }

    private void DisplayGeneralInfo()
    {
        // Find the health property for display.
        EditorGUILayout.PropertyField(serializedObject.FindProperty("health"));
    }
}