After wondering all beauties of this mass production design, I started desoldering the microcontroller and soldering some header pins instead:

Then comes the nucleo board:

And finally I drawn some schematics to facilitate programming:

And of course, add some hole to the USB cable:

Here a very important note!!! Until program a stable software version, keep the microwave transformer hardware disconnected!

Download the hardware testing code to check if is all functional. If it's not, have a good fault-finding.

Once all hardware works, download the basic functionality code and modify it! :D

The code I've attached here is really basic, doesn't have a clock configuration routine nether power control. It just have the real basic functions.