Where can I find source code of official OS image?

Banana Pi BPI-M2 Ultra / BPI-M2 Berry
1

Where can I find source code of official OS image? http://www.banana-pi.org/m2u-download.html

2

HI: you can reference it . M2U BSP: https://github.com/BPI-SINOVOIP/BPI-M2U-bsp

3

I think this is the kernel, not the OS.