How do i add a enemy multiplier to my wave spawner?

I'm a beginner at coding so i usually watch a lot of youtube tutorials for the stuff in my game, i watched a video by Brackeys on a wave spawner I've been trying to figure out how to add a multiplier so that every time the waves loop, the enemies increase. I also want to know how i could add different enemies into the waves.

 public class Spawner : MonoBehaviour 
 {
     public enum SpawnState { SPAWNING, WAITING, COUNTING } 
     [System.Serializable]
     public class Wave
     {
         public string name;
         public Transform enemy;
         public int count;
         public float rate;
     }
     public Wave[] waves;
     private int nextWave = 0;
 
     public Transform[] spawnPoints;
 
     public float timeBetweenWaves = 5f;
     private float waveCountdown;
     public int waveMultiplier = 2;
 
     private float searchCountdown = 1f;
 
     private SpawnState state = SpawnState.COUNTING;
 
     void Start()
     {
         Debug.LogError("No spawn points referrenced.");
 
         waveCountdown = timeBetweenWaves;
     }
 
     void Update()
     {
         if (state == SpawnState.WAITING)
         {
             // Check if enemies are still alive
             if (!EnemyIsAlive())
             {
                 // Begin a new round
                 WaveCompleted();
             }
             else
             {
                 return;
             }
         }
 
         if (waveCountdown <= 0)
         {
             if (state != SpawnState.SPAWNING)
             {
                 // Start spawning wave
                 StartCoroutine(SpawnWave(waves[nextWave]));
             }
         }
         else
         {
             waveCountdown -= Time.deltaTime;
         }
     }
     void WaveCompleted()
     {
         Debug.Log("Wave Completed!");
         state = SpawnState.COUNTING;
         waveCountdown = timeBetweenWaves;
 
         if (nextWave + 1 > waves.Length - 1)
         {                
             nextWave = 0;                
             Debug.Log("All Waves Complete, LOOPING");
         }
         
         else
         {
             nextWave++;
         }
     }   
     bool EnemyIsAlive()
     {
         searchCountdown -= Time.deltaTime;
         if (searchCountdown <= 0f)
         {
             searchCountdown = 1f;
             if (GameObject.FindGameObjectWithTag("Enemy") == null)
             {
                 return false;
             }
         }    
         return true;
     }
 
     IEnumerator SpawnWave(Wave _wave)
     {
         Debug.Log("Spawning Wave: " + _wave.name);
         state = SpawnState.SPAWNING;
 
         // Spawn
         for (int i = 0; i < _wave.count; i++)
         {
             SpawnEnemy(_wave.enemy);
             yield return new WaitForSeconds(1f / _wave.rate);
         }    
         state = SpawnState.WAITING;
 
         yield break;
     }
 
     void SpawnEnemy(Transform _enemy)
     {
         
         Debug.Log("Spawning Enemy: " + _enemy.name);
         Transform _sp = spawnPoints[Random.Range(0, spawnPoints.Length)];
         Instantiate(_enemy,_sp.position, _sp.rotation);    
     } 
 }