STM32 Why can only serial port 1-UART1 download the program?

Q&A:

By default, the STM32 MCU can only use serial port 1 (ie UART1) to download programs. Other UART serial ports will not work. Because the ISP bootloader is only done for serial port 1 in the firmware. If you want to use other serial ports to download programs, you need to modify the bootloader