Scale a unity background to full size in portrait mode, and take about 1/3rd in landscape mode

BackgroundScaler.cs

using Sirenix.OdinInspector;
using UnityEngine;

namespace ADG.Scripts.Runtime
{
    [ExecuteAlways]
    [RequireComponent(typeof(RectTransform))]
    public class BackgroundScaler : MonoBehaviour
    {
        [SerializeField, Required, BoxGroup("References")]
        private RectTransform _rectTransform;
        
        private void Reset()
        {
            if (_rectTransform == null)
                _rectTransform = GetComponent<RectTransform>();
        }

        private void Update()
        {
            UpdateLayout();
        }

        private void UpdateLayout()
        {
            if (Screen.width - 100 > Screen.height) // Landscape
            {
                _rectTransform.anchorMin = new Vector2(0.25f, 0f); // left bound (1/4 from left)
                _rectTransform.anchorMax = new Vector2(0.75f, 1f); // right bound (1/4 from right)
            }
            else // Portrait
            {
                _rectTransform.anchorMin = new Vector2(0f, 0f);
                _rectTransform.anchorMax = new Vector2(1f, 1f);
            }

            _rectTransform.offsetMin = Vector2.zero;
            _rectTransform.offsetMax = Vector2.zero;
        }
    }
}