Technical differences aside:

RIOT is the only mentioned OS with a copyleft license (well, LGPLv2.1), trying to create a FOSS alternative in the embedded/IoT space.

With permissive licenses for embedded software, open source usually ends at the factory.

I'm a big NuttX lover, this is exactly what I was looking for microcontrollers. After spending many years using uClinux , FreeRTOS and others.

NuttX is a kind of Linux-like RTOS. It is very easy to use, as you can see on my video tutorials: www.youtube.com/c/NuttXChannel

Unfortunately using a GPL/LGPL is not an option for embedded system because companies want to keep part of their devices closed.