How to spawn enemies at different locations and avoid overlapping each other

Just to clarify, you just don't want two enemies spawning at the same location right? Just making sure I understand your problem.

I've recently did an implementation of this. Basically whenever I instantiate my prefab, I store it in a list of existing instances ( instances.Add(newInstance);), and when it "dies", I remove it from this list. Then before instantiating I first generate the position, and go through the list of instances to check if the position overlaps. If it does, I generate a new position. However, after a set number of tries I just stop, because if there is no space, I don't want to get into an infinite loop.
In code:

 public class SpawnManager : MonoBehaviour {
 
     [SerializeField] private AsteroidLogic[] asteroidPrefabs = null; // assign asteroid prefabs to it from Editor
     [SerializeField] private maxPositionRetries = 5;
 
     private List<AsteroidLogic> existingAsteroids = new List<AsteroidLogic>();
     public void AddAsteroid (AsteroidLogic value) { existingAsteroids.Add(value); }
     public void RemoveAsteroidLogic (AsteroidLogic value) { existingAsteroids.Remove(value); }
 
     public void Spawn () {
         AsteroidLogic asteroidPrefab = asteroidPrefabs[Random.Range(0, asteoidPrefabs.Length)];
         // probably check if the selected asteroidPrefab is not null!
 
          int numTries = 0;
          do {
              // generate a position, and check if it valid
              Vector3 position = new Vector3(Random.Range(-7.0f, 7.0f), 7, 0);
              if (IsValidPosition(position, asteroidPrefab.transform.localScale)) { // assign the second parameter (size) correctly, for example from the extents of the asteroid's collider, or get it from the AsteroidLogic
                  AsteroidLogic newAsteroid = Instantiate(asteroidPrefab, position, Quaternion.identity);
                  newAsteroid.SetSpawnManager(this);
                  break; // new asteroid generated, break out of loop
              }
          } while (numTries++ < maxPositionRetries);
     }
 
     private bool IsValidPosition (Vector3 positionToCheck, Vector3 objectSize) {
         for (int i = 0; i < existingAsteroids.Length; ++i) {
             AsteroidLogic asteroid = existingAsteroids[i];
             Vector3 asteroidPosition = asteroid.transform.position;
             Vector3 asteroidSize = asteroid.transform.localScale; // assign this correctly, e.g. from the collider's extents
 
              Vector3 positionDifference = positionToCheck - asteroidPosition;
              Vector3 sizeSum = objectSize + asteroidSize;
              // if the positions are less than the sum of the extents, they overlap; also check for z coordinate if you change that
              if (Mathf.Abs(positionDifference.x) < sizeSum.x && Mathf.Abs(positionDifference.y) < sizeSum.y) { return false; } 
         }
         return true;
     }
 
 }
 
 public class AsteroidLogic : MonoBehaviour {
 
     private SpawnManager spawnManager;
 
     // automatically register itself when the manager is set
     public void SetSpawnManager (SpawnManager value) {
         spawnManager = value;
         spawnManager.AddAsteroid(this);
     }
 
     // automatically unregister itself when it gets destroyed
     private void OnDestroy () {
         if (spawnManager != null) { spawnManager.RemoveAsteroid(this); }
     }
 
 }