system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How the data transits online from USA to EU (for example)

I'm familiar with the principle of bits to convey data (with copper cables first then optical fibers), and I know 99% of international traffic comes from cables in the ocean but I would like to know how everything turns out to work when, say you send an e-mail from London to New-York, how can the ridiculous amount of data in the world transit through a few cables?
Hope it's not too complex to be explained.
gpt
There are routers. Routers are made to connect to each other and to your regular everyday computers. They keep a list of who they are directly connected to.

So when you send a message, it gets split into little chunks, goes to your local router, who then passes it to the next router, who does the same, until it gets to the destination.

There is a pathfinding algorithm that finds the best route for each little chunk to take. They each have an id so that the pieces can be put back together in the right order. Each chunk is put in a special packet so that routers can tell where and how to deliver it.

They don't all take the same path to the destination. So one chunk could take the undersea cable, another through satellite, and another going through cables to Africa, then to Europe. 

As others have said, there are a lot of cables and they can carry a lot of data, and relatively speaking an email is not a lot of data.