FindNearest.cs

using UnityEngine;

public class FindNearest : MonoBehaviour
{
    private Vector3 nearestTargetPosition;

    public void Update()
    {
        // Find nearest Target.
        // When comparing distances, it's cheaper to compare 
        // the squares of the distances because doing so 
        // avoids computing square roots.
        float nearestDistSq = float.MaxValue;
        foreach (var targetTransform in Spawner.TargetTransforms)
        {
            Vector3 offset = targetTransform.localPosition - transform.localPosition;
            float distSq = offset.sqrMagnitude;
            if (distSq < nearestDistSq)
            {
                nearestDistSq = distSq;
                nearestTargetPosition = targetTransform.localPosition;
            }
        }

        Debug.DrawLine(transform.localPosition, nearestTargetPosition);
    }
}