First of all a lot of data is cached locally. It is unlikely the data is transferred just for you from Europe to the US. Each cable consists of multiple fiber optic wires. each wire can handle multiple colors and each color can handle 10s of gigabit/s per second. Depending on the distance 400gbit/s per fiber is not unheard of. 100s of these fibers per cable gives a lot of capacity. But again your youtube video will not be streamed on demand from Europe to the US it will be there already.