Django App Plugins
Django app plugins allow extension by automatically adding PIP-installed django apps to the application’s INSTALLED_APPS list. It also allows the app’s settings, URLs, and signals to be recognized by the Django project.
Creating an independently deployed micro-service to add functionality to the platform. A service like this would generally use APIs or Events to communicate with other parts of the platform.
Making a git “fork” of one of the Open edX Platform’s repositories to arbitrarily change code in it. The customization of last resort. Note, making forks to do work you intend to merge upstream is a perfectly valid workflow; this isn’t talking about that, but rather, using forking to make modifications you have no intention of being able to merge upstream.