Baudrate is set to 9600 automatically even if specifying a different value
The whole concept of Baud Rate with USB communication is completely meaningless.  There is no such thing as "baud rate" over USB.

What there is, and what you are confusing with "baud rate" is a configuration item which the host can send to the device which is a "I would like you to communicate with other devices at this speed" configuration item.

This is intended for use with USB to UART adaptors where it is used to configure the baud rate of a physical UART interface for downstream communication.

When you set the baud rate in your code it is ignored, since it has no meaning.

When you set the baud rate on your computer it sends it as this configuration value, which you can then retrieve, should you have an interest in that value, using the Serial.baud() function.