Degree Unit Requirements?????

Hello all. I am a senior at a dual college high school. I’ve completed all of my general ed courses while in high school so when I go to college, I only need to take the major requirements which turns out to be around 30 units(depending on certain classes I decide to take). However, to graduate, I am aware that you need at a minimum 120 units.
I have 60 that will transfer from my current community college and plus the 30 for my major, that leaves another 30 free units. Are these unit supposed to be used on elective courses in your major or can they be used for any courses?
If they can be used for any courses, would it then be possible to double major or minor then and still graduate in two years? Because the way I look at it now, I would be able to graduate in one year except for the 120 unit requirement. So then during that extra year (~30 credits) I can use those units to major in something else? Or am I forgetting something here…