Using Arduino UNO as USB-UART for STM32 Microcontroller (blue pill)

Arduino boards with a USB port (some boards do not have a USB port) already have a USB-UART converter based on the CH340 or FTDI chip, or ATmega, etc. The USB bus of these chips is connected to the USB port, and the UART bus is connected to TX and RX outputs board Arduino.

All you need to do to operate the Arduino board as a USB-UART converter is to disconnect its controller from the UART bus or make it not respond to signals passing through this bus. You can: 

1) do this by connecting the RESET pin to the Arduino with GND pin.

OR

2) upload such a sketch to Arduino:

void setup() 
 { 
   pinMode(0, INPUT); 
   pinMode(1, INPUT); 
 } 
 void loop() 
 {}