I am trying to make a grendade and cant figure out how to apply the damage after it has exploded

Questions & Answers
1

public GameObject explosionEffect;
public float delay = 3f;

public int damageAmount = 80;

public float explosionForce = 10f;
public float radius = 20f;

public void Start()
{
    Invoke("Explode", delay);
}

public void Explode()
{
    Collider[] colliders = Physics.OverlapSphere(transform.position, radius);

    foreach (Collider near in colliders)
    {
        Rigidbody rig = near.GetComponent<Rigidbody>();

        if (rig != null)
            rig.AddExplosionForce(explosionForce, transform.position, radius, 1f, ForceMode.Impulse);
    }

    Instantiate(explosionEffect, transform.position, transform.rotation);
    Destroy(gameObject);

}

private void OnTriggerEnter(Collider other)
{
    PlayerHealth player = other.GetComponent<PlayerHealth>();
    
    if (player != null)
    {
        player.AlterHealth(-damageAmount);
    }

    enemyHealth enemy = other.GetComponent<enemyHealth>();

    if (enemy != null)
    {
        enemy.AlterHealth(-damageAmount);
    }
}

}

2

public void Explode()
{
Collider colliders = Physics.OverlapSphere(transform.position, radius);
foreach (Collider near in colliders)
{
Blow(near);
Damage(near);
}
Instantiate(explosionEffect, transform.position, transform.rotation);
Destroy(gameObject);
}

 private void Blow(Collider other)
 {
     Rigidbody rig = near.GetComponent<Rigidbody>();
     if (rig != null)
         rig.AddExplosionForce(explosionForce, transform.position, radius, 1f, ForceMode.Impulse);
 }

 private void Damage(Collider other)
 {
     PlayerHealth player = other.GetComponent<PlayerHealth>();
     
     if (player != null)
         player.AlterHealth(-damageAmount);

     enemyHealth enemy = other.GetComponent<enemyHealth>();
     if (enemy != null)
         enemy.AlterHealth(-damageAmount);
 }
3

I assume you want everyone in the radius to be affected, in that case you should apply the damage in the Explode method. If players only have 1 collider, then you can just apply the damage where you use AddExplosionForce.
If players have multiple colliders, add the already affected Rigidbodies to a List (because most likely you only want to affect all players once instead of the number of colliders they have).
Solution with multiple colliders (it works with a single collider too):

	public void Explode()
	{
		Collider[] colliders = Physics.OverlapSphere(transform.position, radius);
		List<Rigidbody> affectedRigs = new List<Rigidbody>();

		foreach (Collider near in colliders)
		{
			//I recommend using attachedRigidbody instead of GetComponent because
			//it finds the Rigidbody even if it's among the parent objects
			Rigidbody rig = near.attachedRigidbody;
			if (rig != null && !affectedRigs.Contains(rig))
			{
				rig.AddExplosionForce(explosionForce, transform.position, radius, 1f, ForceMode.Impulse);
				PlayerHealth player = rig.GetComponent<PlayerHealth>();
				if (player != null)
				{
					player.AlterHealth(-damageAmount);
				}
				affectedRigs.Add(rig);
			}
		}

		Instantiate(explosionEffect, transform.position, transform.rotation);
		Destroy(gameObject);
	}