Ergomax's firmware is based on QMK, the (most?) popular firmware for custom keyboards.
The firmware is hosted on a separate repository (fork of QMK) in the ergomax-keyboard branch: LouWii/qmk_firmware
To read more, go to the Ergomax folder qmk/keyboards/ergomax
kbfirmware.com is a nice tool that makes it easy to create a firmware based on QMK. You just have to setup your layout, enter the settings, enter the layout and layers, enter macros and even custom code, and then the tool compiles a firmware for you, or you can even download the source files if you want to.
I've created a file for the left side and right side so you don't have to setup everything in the tool. Just upload ErgoMaxLeft/kfb-layout.json or ErgoMaxRight/kfb-layout.json and add your own customizations.