I've seen FreeRTOS being used besides QNX and other proprietary RTOSes in automotive applications. Do you know how similar or different FreeRTOS and QNX are?
Completely different under the hood, not to be compared, but both have their uses.
I think FreeRTOS is monolithic, isn't it?
Yes it is. It's more or less a scheduler library, that you link together with all the other code you have to get a microcontroller system image. There are no processes in FreeRTOS, just tasks (threads).