I don't know that I'd recommend learning FreeRTOS for fun because you should basically never be using a traditional RTOS if you can possibly avoid it. Unless you really like reinventing stuff that you get "for free" on Linux, there are remarkably few advantages beyond certifiability (which FreeRTOS in particular doesn't get you), easier porting (just buy something that actually runs Linux) and lower resource use (more work to actually achieve!).
As for Linux, your first stop should be the bootlin materials [1]. They're fairly comprehensive and regularly updated.