Macros are in MQ2's macro language. All macros, even Kiss, are open source as the macro is just a text file. So you can read/edit any macro with a text or code editor like Notepad++. Examples: KissAssist, IHC's macros, cskillup, the various PoK Hatching a Plan automaters.
Plugins are are in c++. These are mainly closed source, though some older broadly used ones are available on the MQ2 paid forums and various gits. Examples: MQ2Melee, MQ2Berserker, MQ2EQWire and such.
This thread has some
good info about the codes and getting started. A lot of us kind of started out just tinkering with the macros we used and wound up learning to program.