That is a very broad question. The serial data sent by the I2C serial protocol is commonly used for short range microcontroller to auxillary chip signaling.
Do you mean to ask "how is I2C used in a communications satellite?". We need some clarification.
Have you looked at any of the links? The I2C is a serial communication protocol used on printed circuit boards so a microcontroller can send commands and get responses from peripheral chips. The microcontroller is the Master. The bus is a trace on the PCB.
I have asked the only authority on the subject of satellites electrical inner workings that I know, and he said, "American satellites, that we design, do not use I2C for long distance communications or I2C for inter component communications. However, other countries or private companies may."
So, I inferred from this that it would be used in the same way in a satellite that it would on your bench.