Internals: Miscellaneous Topics - Macaulay2/M2 GitHub Wiki
Work in progress. Contributors: Michael Burr, Anton Leykin, (your-name-here)
Doxygen
This documentation isn't complete, but it does contain some useful information. The main goal of the doxygen documentation is for the internals. In particular, the class structure and inheritance is described here.
Branches
The differences between current branches of M2 include memory allocation schemes (related to garbage collection issues) and build methods.