Correct, the green board is the PCB and the microcontroller is underneath that black dot. You won't be able to program it without some deeper knowledge in circuits and Microcontroller coding. Realistically your best bet is going to be swapping out the current microcontroller for a product like a pro micro or similar Microcontroller (MCU) that already has good documentation and support.
You could try mapping the matrix like others have done
here.. That linked thread will give you all the information (outside of a specific step by step) of what it takes to do what you are wanting to do. But my honest opinion, it's a membrane keyboard and not worth the investment and time. If you are curious about learning keyboard hardware and flashing custom macropads, etc. I would just buy this
Adafruit Macropad.
It's based off the RP2040, which isn't the most popular microcontroller in the keyboard world, but is gaining traction. Adafruit also has TONS of educational articles and guides
here. I am also trying to teach myself microcontroller design. It's a steep hill with very little in the way of introductory projects. Good luck.
