How to create cube made of cubes that are all fixed together?

Just give them all rigidbodies, set them to kinematic, so they will not be affected by outside forces. Then, parent them all to an empty gameobject, which will serve as their pivot point (the object you will move to move the cube). Then, in a script attached to your pivot point (we can call this object cube), make an array of gameobjects, which will be your 1x1x1 cubes. If you haven't used arrays before, here's a good video explaining them https://www.youtube.com/watch?v=RQ0JHMGiobo TL:DR a gameobject array is a variable that stores multiple gameobjects, arrays can also store other data types, like box colliders, or floats. Then, you can loop through that array on a collision, like so

 void OnCollisionEnter2D(Collision2D col)
 {
        for each(GameObject cube in cubes) //cubes is the array of gameobjects
        {
                 cube.GetComponent<Rigidbody>().isKinematic = false;
        }
 }

this should do the trick, let me know if you have any more questions