Using TX/RX serial pins to communicate over USB
There is no &quot;built-in USB&quot; on the Nano. There is only the FT232RL chip which talks USB. That chip is connected directly to the TX/RX pins inside the board.
You are already communicating over the TX/RX pins through the FT232RL to the USB.
Yes, you could use your own FT232RL (or FT232-like) board to communicate between USB and the TX/RX pins, but that will be exactly the same as using the one built in to the board - but without the convenience, and with the addition of more wires and expense.  So pointless, unless the current USB connection is damaged in some way.