Arduino for STM32

yes.
Now there are several versions of the bootloader, the older of them stays in bootloader for 3 seconds and jumps to the user code only after that time period to give chance the uploader app to upload.
The fastboot version does as I explained before, instead waiting it checks for some conditions and jumps directly to the user code if the case. In this case you have to set jumper BOOT1 to 1 or user USB serial.

N5BOC wrote: ↑Fri May 08, 2020 3:08 amNo one seems to just use an ST-LINK. I don't understand why not really.

it is ok to just use st-link (v2) or even for that matter usb-uart. the slight benefit (with stm32f103) with the usb e.g. dfu (roger's) bootloader is that you can make do with just a single usb cable, no extra st-link v2 or usb-uart dongles is needed to upload firmware. using st-link or usb-uart is sometimes more convenient, dependent on circumstances and the use case.