serial data over usb

When i send a serial data from the serial monitor to the arduino, my pc converts the serial data to usb? I know that when i send data from arduino to pc there is a serial to usb converter in the board that makes the conversion, but what happens in the reverse side? if i send the serial data from my pc (from serial monitor or matlab for example) to arduino who in fact converts the serial data to usb?

The USB subsystem in your computer.

Guess what the "S" in USB stands for. 8^)

iSelector: if i send the serial data from my pc (from serial monitor or matlab for example) to arduino who in fact converts the serial data to usb?

Starting from MATLAB working towards your Uno...

The operating system provides an interface to MATLAB that essentially fools MATLAB into believing it is communicating through a serial port. MATLAB calls a function on that interface requesting data to be sent through that make-believe serial port.

The request to send serial data goes through minor changes becoming a request for a USB data transfer.

The request for a USB data transfer is routed to the appropriate USB bus controller within your computer. The USB bus controller schedules then sends the data to a USB end-point. The end-point in this case is the ATmega16U2 on your Uno.

The ATmega16U2 makes some minor changes to the USB data then transmits that data through its serial port to the ATmega328P.

In other words, it is all USB smoke-and-mirrors until you get to the three wires connecting the ATmega16U2 to the ATmega328P.