Check out example codes for "unity sprite disappears when using transform.lookat". It will help you in understanding the concepts better.

Code Example 1

// Do not use "transform.LookAt" for 2D 
// Instand use something similar to this

public static void LookAt2D(this Transform transform, Vector2 target)
    var direction = target - (Vector2)transform.position; // Get the direction
    var angle = Mathf.Atan2(direction.y, direction.x) * Mathf.Rad2Deg; // Convert to angle
    transform.rotation = Quaternion.AngleAxis(angle - 90, Vector3.forward); // Finally rotate the GameObject

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