Quaternion.LookRotation 例

Aim1.cs

//目標の方を向く例 1
public class Aim : MonoBehaviour
{
    [SerializeField]
    private GameObject _target;

    void Update()
    {
        var directionToFace = this._target.transform.position - this.transform.position;

        //y固定 にしたい場合
        directionToFace.y = 0f;

        this.transform.rotation = Quaternion.LookRotation(directionToFace);

        Debug.DrawRay(this.transform.position, directionToFace, Color.green);
    }
}

Aim2.cs

//目標の方を向く例 2
public class Aim : MonoBehaviour
{
    [SerializeField]
    private GameObject _target;

    [SerializeField]
    private float _speed = 5f;

    void Update()
    {
        var directionToFace = this._target.transform.position - this.transform.position;
        
        //y固定 にしたい場合
        //directionToFace.y = 0f;

        var targetRotation = Quaternion.LookRotation(directionToFace);
        this.transform.rotation = Quaternion.Slerp(this.transform.rotation, targetRotation, Time.deltaTime * this._speed);

        Debug.DrawRay(this.transform.position, directionToFace, Color.green);
    }
}

Debug.DrawRay(this.transform.position, directionToFace, Color.green);