collision detection between two players in unity code example

Example 1: unity how to make two objects not collide

void OnCollisionEnter(Collision collision)
  {
      if (collision.gameObject.tag == "theobjectToIgnore")
      {
          Physics.IgnoreCollision(theobjectToIgnore.collider, collider);
      }

Example 2: check for collision unity c#

void OnCollisionEnter(Collision col) { }

Example 3: how to do collision tag in unity

void OnColliSionEnter2D(Collision col){
   if (col.gameobject.tag == "Enemy"){
        Destroy(gameObject)
        }
}