Each cortex ARM microcontroller is equipped with a bootloader which is built into the chip during the production. So as explained here you can program your chip over a UART port with an USB to TTL UART converter if you like so. An alternative to the ST-LINK utility is the newer STM32CubeProgrammer available also on Mac and Linux. Of course a programmer is a more convenient piece of hardware and usually it enables also to debug your program.
