Check out example codes for "unity how to tell when a gameobject is colliding". It will help you in understanding the concepts better.

Code Example 1

void OnCollisionEnter(Collision collision){
        //Check for a match with the specified name on any GameObject that collides with your GameObject
        if (collision.gameObject.name == "MyGameObjectName")
        {
            //If the GameObject's name matches the one you suggest, output this message in the console
            Debug.Log("Do something here");
        }

        //Check for a match with the specific tag on any GameObject that collides with your GameObject
        if (collision.gameObject.tag == "MyGameObjectTag")
        {
            //If the GameObject has the same tag as specified, output this message in the console
            Debug.Log("Do something else here");
        }
    }

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