Pressing and releasing the BOOTLOADER button will cycle the power and start the MCU with 1k pull-up on the USB-P line. This will start the built in USB/Serial bootoader. External BOOT input is provided for triggering via DTR or other active low signal.
Take a look here for similar circuit for the CH559 series:
https://github.com/hexeguitar/CH55x-HW-reboot
Python programming script for both USB and serial port can be found here:
https://github.com/hexeguitar/CH55x_python_flasher
(c) 11.2020 by Piotr Zapart
www.hexefx.com