if you want to learn embedded system development that runs on some embedded operating system, I recommend starting with Building Embedded Linux Systems: Concepts, Techniques, Tricks, and Traps eBook
To learn real time operating system development, get started with Coursera | Real Time Systems, not only this course talks about most of the rtos concepts but also introduces to FreeRTOS, then you can buy some hardware to which FreeRTOS has been ported and started experimenting with it.
If you have time and money I suggest doing Embedded Systems specialization from UCLA or UCSD, they are fully online.