Ask HN: Embedded Projects to Learn From?

I'm currently at a small company and often find myself looking outside for solutions and techniques in the embedded software space. The areas I'm interested in are C, testing and testability, architecture and portability, real-time control, build systems and CI. Leaving books and Stack Overflow aside, the value of projects is in their demonstration: the maturity/immaturity, advantages/drawbacks of particular tools and techniques.

The following projects have been of great pedagogical use to me:

https://github.com/google/makani (Bazel, C, testing, architecture, real-time control, code-generation)

https://github.com/odriverobotics/ODrive (real-time control, code-generation, C++)

https://github.com/FreeRTOS/FreeRTOS-Kernel (architecture, C)

https://github.com/espressif/esp-idf (architecture, C, CMake, testing)

Which embedded software projects have you learned from and what have you learned from them?