If A and B both send contined digital signals to C through a Cat 5 Cable. How can C tell which singal is from A and which singal is from B?
It can’t. That’s why all messages sent over a network contains a sender id.
Different networks have different ways of dealing with collisions (two senders sending at the same time). The most common one these days is probably Ethernet, which is basically just a shouting match. 
Shouting match with a preamble acting as a lock, more or less.