Is the real time OS used on the Meadow board open source anywhere?
The various components are open source but at the moment the current implementation is closed source.
On the STM32, the low level RTOS is based on NuttX with Mono sitting on top of this. W have customised NuttX and Mono to make this ecosystem possible.
The ESP32 runs FreeRTOS with our custom software allowing the ESP32 to provide network services to NuttX.