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?