The ST bootloader is programmed in the system memory of stm32 and locked to prevent users from erasing and writing. This bootloader is used for application programming, such as punctual atom's usb-to-serial hardware and flymcu software, through which the program can be programmed to flash.
The bootloader that most people say refers to the bootloader program written by themselves and programmed to the starting position of the flash (stm32 is 0x8000000), and the app program is updated through this program.