damage system

so i have a gun script and a custom bullet script does any one know how i can make a damage system like the enemy will have 100 health and not a one shot

this is the gun script //bullet force public float shootForce, upwardForce;

 //Gun stats
 public float timeBetweenShooting, spread, reloadTime, timeBetweenShots;
 public int magazineSize, bulletsPerTap;
 public bool allowButtonHold;

 int bulletsLeft, bulletsShot;

 //Recoil
 public Rigidbody playerRb;
 public float recoilForce;

 //bools
 bool shooting, readyToShoot, reloading;

 //Reference
 public Camera fpsCam;
 public Transform attackPoint;

 //Graphics
 public GameObject muzzleFlash;
 public TextMeshProUGUI ammunitionDisplay;

 //bug fixing :D
 public bool allowInvoke = true;

 private void Awake()
 {
     //make sure magazine is full
     bulletsLeft = magazineSize;
     readyToShoot = true;
 }

 private void Update()
 {
     MyInput();

     //Set ammo display, if it exists :D
     if (ammunitionDisplay != null)
         ammunitionDisplay.SetText(bulletsLeft / bulletsPerTap + " / " + magazineSize / bulletsPerTap);
 }
 private void MyInput()
 {
     //Check if allowed to hold down button and take corresponding input
     if (allowButtonHold) shooting = Input.GetKey(KeyCode.Mouse0);
     else shooting = Input.GetKeyDown(KeyCode.Mouse0);

     //Reloading 
     if (Input.GetKeyDown(KeyCode.R) && bulletsLeft < magazineSize && !reloading) Reload();
     //Reload automatically when trying to shoot without ammo
     if (readyToShoot && shooting && !reloading && bulletsLeft <= 0) Reload();

     //Shooting
     if (readyToShoot && shooting && !reloading && bulletsLeft > 0)
     {
         //Set bullets shot to 0
         bulletsShot = 0;

         Shoot();
     }
 }

 private void Shoot()
 {
     readyToShoot = false;

     //Find the exact hit position using a raycast
     Ray ray = fpsCam.ViewportPointToRay(new Vector3(0.5f, 0.5f, 0)); //Just a ray through the middle of your current view
     RaycastHit hit;

     //check if ray hits something
     Vector3 targetPoint;
     if (Physics.Raycast(ray, out hit))
         targetPoint = hit.point;
     else
         targetPoint = ray.GetPoint(75); //Just a point far away from the player

     //Calculate direction from attackPoint to targetPoint
     Vector3 directionWithoutSpread = targetPoint - attackPoint.position;

     //Calculate spread
     float x = Random.Range(-spread, spread);
     float y = Random.Range(-spread, spread);

     //Calculate new direction with spread
     Vector3 directionWithSpread = directionWithoutSpread + new Vector3(x, y, 0); //Just add spread to last direction

     //Instantiate bullet/projectile
     GameObject currentBullet = Instantiate(bullet, attackPoint.position, Quaternion.identity); //store instantiated bullet in currentBullet
     //Rotate bullet to shoot direction
     currentBullet.transform.forward = directionWithSpread.normalized;

     //Add forces to bullet
     currentBullet.GetComponent<Rigidbody>().AddForce(directionWithSpread.normalized * shootForce, ForceMode.Impulse);
     currentBullet.GetComponent<Rigidbody>().AddForce(fpsCam.transform.up * upwardForce, ForceMode.Impulse);

     //Instantiate muzzle flash, if you have one
     if (muzzleFlash != null)
         Instantiate(muzzleFlash, attackPoint.position, Quaternion.identity);

     bulletsLeft--;
     bulletsShot++;

     //Invoke resetShot function (if not already invoked), with your timeBetweenShooting
     if (allowInvoke)
     {
         Invoke("ResetShot", timeBetweenShooting);
         allowInvoke = false;

         //Add recoil to player (should only be called once)
         playerRb.AddForce(-directionWithSpread.normalized * recoilForce, ForceMode.Impulse);
     }

     //if more than one bulletsPerTap make sure to repeat shoot function
     if (bulletsShot < bulletsPerTap && bulletsLeft > 0)
         Invoke("Shoot", timeBetweenShots);
 }
 private void ResetShot()
 {
     //Allow shooting and invoking again
     readyToShoot = true;
     allowInvoke = true;
 }

 private void Reload()
 {
     reloading = true;
     Invoke("ReloadFinished", reloadTime); //Invoke ReloadFinished function with your reloadTime as delay
 }
 private void ReloadFinished()
 {
     //Fill magazine
     bulletsLeft = magazineSize;
     reloading = false;
 }

}

this is the custom bullet public class CustomBullet : MonoBehaviour { //Assignables public Rigidbody rb; public GameObject explosion; public LayerMask whatIsEnemies;

 //Stats
 [Range(0f,1f)]
 public float bounciness;
 public bool useGravity;

 //Damage
 public int explosionDamage;
 public float explosionRange;
 public float explosionForce;

 //Lifetime
 public int maxCollisions;
 public float maxLifetime;
 public bool explodeOnTouch = true;

 int collisions;
 PhysicMaterial physics_mat;

 private void Start()
 {
     Setup();
 }

 private void Update()
 {
     //When to explode:
     if (collisions > maxCollisions) Explode();

     //Count down lifetime
     maxLifetime -= Time.deltaTime;
     if (maxLifetime <= 0) Explode();
 }

 private void Explode()
 {
     //Instantiate explosion
     if (explosion != null) Instantiate(explosion, transform.position, Quaternion.identity);

     //Check for enemies 
     Collider[] enemies = Physics.OverlapSphere(transform.position, explosionRange, whatIsEnemies);
     for (int i = 0; i < enemies.Length; i++)
     {
         //Get component of enemy and call Take Damage

         //Just an example!
         ///enemies[i].GetComponent<ShootingAi>().TakeDamage(explosionDamage);

         //Add explosion force (if enemy has a rigidbody)
         if (enemies[i].GetComponent<Rigidbody>())
             enemies[i].GetComponent<Rigidbody>().AddExplosionForce(explosionForce, transform.position, explosionRange);
     }

     //Add a little delay, just to make sure everything works fine
     Invoke("Delay", 0.05f);
 }
 private void Delay()
 {
     Destroy(gameObject);
 }

 private void OnCollisionEnter(Collision collision)
 {
     //Don't count collisions with other bullets
     if (collision.collider.CompareTag("Bullet")) return;

     //Count up collisions
     collisions++;

     //Explode if bullet hits an enemy directly and explodeOnTouch is activated
     if (collision.collider.CompareTag("Enemy") && explodeOnTouch) Explode();
 }

 private void Setup()
 {
     //Create a new Physic material
     physics_mat = new PhysicMaterial();
     physics_mat.bounciness = bounciness;
     physics_mat.frictionCombine = PhysicMaterialCombine.Minimum;
     physics_mat.bounceCombine = PhysicMaterialCombine.Maximum;
     //Assign material to collider
     GetComponent<SphereCollider>().material = physics_mat;

     //Set gravity
     rb.useGravity = useGravity;
 }

 /// Just to visualize the explosion range
 private void OnDrawGizmosSelected()
 {
     Gizmos.color = Color.red;
     Gizmos.DrawWireSphere(transform.position, explosionRange);
 }

}

thanks for your time.