Creating a gameObject in Unity is called Instantiation and the syntax for it is:
You can further extend this command by specifying the location where you want to instantiate the object and its rotation upon instantiation.
Instantiate(gameObjectPrefab, transform.position, Quaternion.identity);
Quaternion.Identity is used when you do not want to mess with the rotation of the object upon instantiation.
This command is useful in many cases; for example in our space project, we used it to create lasers upon hitting the space button.
Destroy GameObject is used when you want to destroy an enemy for example, destroy a pickup after using it, or it is very helpful to use it to declutter the hierarchy during game play.
The syntax is
Destroy(this.gameObject);//this can be extended to allow the gameObject to be destroyed after a certain delayDestroy(this.gameObject, 5f);
//this will destroy the object after 5 seconds.