Tag & Layer

using UnityEngine;

public class TreasureChest : MonoBehaviour
{
  private void OnTriggerEnter(Collider other)
  {
      // CompareTag does not allocate a string — better than ==
      if (other.CompareTag("Player"))
      {
          Debug.Log("Player collected the treasure chest!");
          gameObject.SetActive(false);
      }
  }
}

// Find every Enemy in the scene by Tag
GameObject[] enemies = GameObject.FindGameObjectsWithTag("Enemy");
foreach (var e in enemies) e.GetComponent<EnemyAI>().Alert();

using UnityEngine;

public class PlayerGroundCheck : MonoBehaviour
{
  [SerializeField] private float checkDistance = 0.1f;

  // Build LayerMask from names — cache it, do not rebuild every frame
  private readonly int _groundMask = LayerMask.GetMask("Ground", "Platform");

  public bool IsGrounded =>
      Physics.Raycast(transform.position, Vector3.down, checkDistance, _groundMask);

  void Update()
  {
      if (IsGrounded)
          GetComponent<Rigidbody>().drag = 5f;   // friction while grounded
      else
          GetComponent<Rigidbody>().drag = 0f;   // no drag in the air
  }

  // Change Layer at runtime (e.g. ghost through walls)
  public void SetGhostMode(bool ghost)
  {
      gameObject.layer = LayerMask.NameToLayer(ghost ? "Ghost" : "Player");
  }
}