Question about using serial monitor communicate with STM32

Not fully true. USART1 can be remapped ( to PB7/PB6 ) but USART2/USART3 can be not ( due to the fact, that on the small 48pin chip on bluepill the according remap pins are not available).
But I don't know if it can be done by core functions or has to be done by hw registers directly.