Accessing objects in C#

This may sound a bit idiot, but... In C#, when am I accessing an object and when am I just taking its value ?

For example:

 Renderer ren = GetComponent<Renderer>();
 /*
 *this sets ren to the GameObject's renderer.ç
 *If I change something in the var it will change also in the renderer.
 */

But, the thing is this:

 Texture2D tex = GetComponent<Renderer>().material.mainTexture ;
 /*
 *This dont set the var as the material's main texture
 *It just duplicates it and puts it into the var,
 *so, if I change something in the var, the main texture won't change
 */

So, how do I know if I'm pointing to an object or just taking its value?