Virtual com ports or USB CDC do not require a baud rate as it's not a real serial port. As mentioned ESP32 has native USB and Arduino/ESP-IDF use USB CDC over this port to communicate with a host computer. Serial.begin() is used for all kinds of serial ports including virtual serial ports. Those libraries probably require a baudrate argument for Serial.begin() which will be ignored. This is probably in the documentation for that function.
If the same function is used on a physical serial port (of which there are a few on ESP32 iirc) the baudrate argument will be used to set the baudrate setting in the peripheral by the library.
The purpose of baud and other control settings for CDC devices is so that they can serve as a pass-through for a real UART that will need to know what those settings are. Those control packets can be ignored if that never happens.
If the same function is used on a physical serial port (of which there are a few on ESP32 iirc) the baudrate argument will be used to set the baudrate setting in the peripheral by the library.