Boot0

All STM32 MCUs includes a GPIO pin labelled Boot0. This pin is used to select the boot mode.

See this page for how to jump to the System bootloader from an application: STM32 Jump to System Memory Bootloader

Hardware design

It is extremely important to pull the Boot0 pin low during reset for normal operation.

If the pin is left unconnected in a hardware design it will "probably work", but it "might" on occasion detect a "high" on the pin and therefore decide to switch to the internal bootloader.

The proper way to deal with it can be seen on my Green Pill design:

The button SW2 is of course optional but while the pull-up resistor on the reset line (as indicated) is optional, the pull-down resistor R5 on Boot0 is not optional.

The strength of the pull-down is not very critical. Anything from 10k to 1M will do the job alright.