Understanding ros 2 navigation stack, white papers and software architecture sketches would help

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I would like to understand the ROS 2 navigation stack and perhaps help in its development.

The repository that you link to is not the "ROS 2 navigation stack" if I understand everything correctly. It seems to be a (quick) port of the "ROS 1 navigation stack" to use ROS 2 infrastructure (it was used as a demo/test case for the state of ROS 2 quite a few months ago).

As such, the design is probably relatively similar to how it was in ROS 1.

The effort to implement a proper ROS 2 based navigation stack may be found at: ros-planning/navigation2.

The design documentation (and other documentation) for that may be found at: ros-planning/navigation2/doc (NOTE: I'm using a permalink here. If you're clicking this link in the future, make sure to read an up-to-date version of the documentation, not this old one).