Why do Computers use Binary

3 things I learned:
1. Binary is a base two system
2. the numbers encode with different voltages
3. There is no base 1 numeral system

2 things i didn't understand
1. What he meant by saying choice is the essence of twoness
2. How everything in computers could be each have an individual binary code.

1 thing I would like to know
1. Does every computer use binary, and if so, do they only use binary to code things or is there different systems as well.