I found the solution here, below. I can confirm this works for anyone else having this problem. You need to connect STM32 MCU to USB for serial monitor and ST-LINK for downloading. In Arduino IDE You can then see two ports and connect to the USB port for monitoring.
You also need to select a different board name variation for serial monitor otherwise it won't connect with the same one (bottom pic).
"ST-Link is not a Serial adapter, it is a programmer. You can't open a Serial Monitor over ST-Link.
to see the Serial prints connect the Blue Pill with USB cable. without bootloader listening on USB you can't upload over USB but it works as Serial (or Serial1?) with Serial Monitor or other terminal program"
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