The MiKTeX Project Page is the place to go, if you are new to MiKTeX.
In short: MiKTeX is a modern C/C++ implementation of TeX & Friends for Windows, macOS and Linux. The MiKTeX source code is documented here: https://docs.miktex.org/hacking/index.html
MiKTeX is also a scalable TeX distribution ("Just enough TeX"):
- you have the option to start with MiKTeX executables and some configuration files
- in the course of authoring your documents, only necessary LaTeX packages, fonts etc. will be downloaded and installed
MiKTeX can be built on Windows and Unix-like (including macOS) systems. Please consult these HOWTOs for platform-specific build instructions:
- https://miktex.org/howto/build-win
- https://miktex.org/howto/build-unx
- https://miktex.org/howto/build-mac
In addition, you can try one of the Dockerized build environments to build MiKTeX:
If you want to understand the MiKTeX source code, have a look at HACKING.md.