There are many many ways in unity for objects to access each other's components, scripts, functions and variables. Start reading about it at these two manual pages:

• Accessing Other Objects
• Accessing Other Components

Also see this answer which gives more detail about creating "dragged references":

How can I access other scripts and their functions?

In your particular case, if you only have a single "Player" at one time, you could use "FindObjectOfType" to find the player script instance at runtime, when the enemy is created. FindObjectOfType is ideal when you know the script you're looking for is the only one of its kind in your scene. It's relatively slow to execute, so we store the reference in a variable in Start() and just use the variable later.

I'm assuming you have a script on your player, and for this example I'm going to assume it's called "PlayerScript". Whatever it's called, you need to use the exact script name where I write "PlayerScript":

This would be in your enemy script:

private PlayerScript player;

void Start() {
    // find the current instance of the player script:
    player = FindObjectOfType(typeof(PlayerScript));
}

You can then access public variables on the player's script, from inside your enemy script, using code like this:

player.health

And call functions on the player's script like this:

player.ApplyDamage(20);

You can also access the Game Object to which the player script is attached:

player.gameObject

And other built-in component references:

player.transform
player.renderer
player.collider
player.rigidbody
//..etc

As well as any other component type which doesn't have a built-in reference:

SomeOtherComponent c = player.GetComponent<SomeOtherComponent>();

(where "SomeOtherComponent" is the name of another component or script)

If there is more than one object of a certain type, and you want to get references to all of them, you can use the very similarly named "FindObjectsOfType" function. This differs in that it returns an Array of the type of object that you specify. For example, if you have 10 enemies in your scene, each with "EnemyScript" attached, you could use:

EnemyScript[] enemies = FindObjectsOfType<EnemyScript>();

You now have an array containing references to all the current instances of the enemy scripts. You can then iterate through them using a foreach loop:

foreach (EnemyScript enemy in enemies)
{
    // do whatever with each 'enemy' here
}

What you want to learn about is public properties in C#, getters and setters.