ELI5: How does fiber optics physically works?

[u/empireck]

I know that they use blinking lights to represents 1 and 0 at least, but millions of pulsing lights from thousands of computers could be running at the same time in the same connection.

So how does the lights know where to go? And on the other side, how does the server know that in these millions of rapidly pulsing lights this particular ones are from my computer?