Marlin Firmware is the firmware that is used on Mega Zero printers.
Current releases of the Marlin upstream project have configuration examples for both old (1.1.9) and current (2.0.x) branches for Mega Zero printer.For Mega Zero 2.0, few options related to heated bed should be enabled on top of those examples. Official installation documentation on Marlin can teach you on needed steps on how to use configuration examples and tools to compile your own version of firmware.
Using PlatformIO is the easiest way to build and flash firmware. You can find official Marlin documentation on using PlatformIO on official site. PlatformIO method also includes nice plugin to popular Visual Studio Code IDE.
Official documentation on PlatformIO with Visual Studio Code.