Well, your question is out of the scope of programming at all and even lower then hardware level. This is a question for basic physics and electronics.
Computer doesn't have to understand 1 and 0 because this is already how electornic works, it operates on voltage level already. Imagine a light switch, when it is turned off - there are no light in the room, when you manually press the switch, it is on and there is a light. We, humans, just represent a state of the switch (0 - off, 1 - on) or signal (0 - absence of an electrical signal, 1 - presence of an electrical signal). Low voltage we represent as 0 and high voltage as 1. Read more in this article: Why 1’s and 0’s?
Years ago you should turn switches on/off manually, but now there are transistors. Here is a good short video explaining how tansistor works.