Collision system not working. There are no errors, it just wont register that the projectile hit the player - unity3d

Question

Here is the code in which I was talking about. I can't figure out why my collider isn't working:
using System.Collections.Generic;
using UnityEngine;
public class SphereCollider : MonoBehaviour
{
GameObject obj;
void Awake ()
{
obj = GameObject.FindGameObjectWithTag("Player");
}
// Update is called once per frame
void Update()
{
void OnCollisionEnter(Collision collision)
{
if (collision.gameObject.name == "Player")
{
obj.GetComponent<Health>().health -= 25;
}
}
}
}

Answer 1

Rather than using the code you have, make sure to change some code. This is what you should change: get rid of the obj variable and the awake function. Then instead of finding the object with that name, use tag. Also get rid of Update(){} and take the OnCollisionEnter(){} out of it.
A few more changes are shown in code:
void OnCollisionEnter(Collision collision){
var obj = collision.GameObject;
if (obj.Tag == “Player”){
obj.GetComponent<Health>().health -= 25;
}
}
This will work as long as: the script is called Health, the variable inside Health is called health, and it is set to a float, and the player’s tag is set to Player.

Answer 2

Not sure if the code pasted in the question is just copied over wrong, but OnCollisionEnter() is its own unique method and should not be nested in Update().
If the player will be colliding with this Sphere object often, it is good to cache a component rather than using GetComponent frequently. However I assume in your case this Sphere will be one of many obstacles, so calling the GetComponent in the collision is fine. The issue with your current code is the gameObject you are colliding with might not have the name exactly spelled as Player, which would make your collision check fail. Caching the object that has the tag of Player is a bit redundant when you can just check it inside of the collision check.
I will use your entire snippet of code so there is no confusion
using System.Collections.Generic;
using UnityEngine;
public class SphereCollider : MonoBehaviour
{
void OnCollisionEnter(Collision col)
{
// compare if the collider's gameObjects tag is equal to our target tag (Player)
if (col.gameObject.tag == "Player")
{
// get the component of Health on our gameObject that we collided with that has the tag of Player
obj.GetComponent<Health>().health -= 25;
}
}
}