How does most of the overseas telecommunication take place through satellite or undersea cables ??

Answer 1

Satellites in geosynchronous orbit (remaining over one spot) are up about 22,000 miles, so communication across an ocean using a satellite requires a signal to travel about 44,000 miles.
The same journey through an undersea cable will probably only be about 5,000 miles.
As an exercise, determine how long it takes to send 'hello world' in a telnet session via satellite, and compare the result to the amount of time to send it via undersea cable. Assume your telnet application sends one character at a time and waits for an ACK after each character is sent. Assume your signal travels at the speed of light and all your network equipment amazingly causes no delay.

Now assume you have some application where you can send data in one direction and not require any acknowledgements to be sent (such as broadcast television) or an application where you've tuned it so you require very few acknowledgements (ftp with an insanely large transmit window, or an application where you embed error correction codes in the data to allow the receiver to attempt to detect AND correct transmission errors). In that case, the delays in satellite transmission aren't a big problem.

Answer 2

Undersea cables mostly. Satellites are used but have comparatively limited bandwidth and are more expensive.